having inherited a long-staying animosity
--Both want the chance to kill the other.
--Both hesitate when they unexpectedly come face-to-face with each other because 

...a man who has been brought up under the code of a restraining civilization cannot easily nerve himself to shoot down his neighbor in cold blood and with word spoken, except for an offense against his hearth and honor.

"Nature's own violence overwhelmed them both" as they are each pinioned by the large limbs of a beech tree struck by lightning.
--Each of the two men threaten the other after the accident, declaring that their men will arrive and kill the other, their enemy.
--Both men curse the other and they both "speak with the bitterness of possible defeat before them":

"Death and damnation to you Ulrich von Gradwitz."
"The same to you, Georg Znaeym, forest-thief, game-snatcher."

--Both experience a change of heart regarding their feud after they wait for some time for their parties. Faced with death, they realize the pettiness of their quarrels and change their minds about the gravity of their feud. Then, following some discussion, they agree to put aside any quarrels and differences and, henceforth, to be neighborly toward one another.
--Each man prays privately that his party will arrive so that he can be the gracious one and show "honorable attention to the enemy who had become a friend."
--Both men experience the "hideous fear" of the approaching wolves.

• Differences

--The family of Ulrich von Gradwitz is wealthier than that of Georg Znaeym, for the Znaeyms are considered "petty landowners."
--Because the von Gradwitz family has "wrested" the land in a famous lawsuit from the "illegal possession" of the neighboring Znaeyms, the von Gradwitzes feel entitled to the land; on the other hand, the Znaeyms view the lawsuit as an usurpation of their property.
--When they are injured by the tree, Ulrich has "some drops of blood" in his eyes, but George is "nearly blinded by the blood."
--von Gradwitz considers Znaeym a "forest-thief" and a "game-snatcher," while Znaeym simply hates von Gradwitz for taking the land that he feels was once his family's.
--It is Ulrich who first broaches the idea of reconciliation between them.

This is a great question.  I sympathize with Gerog more than I sympathize with Ulrich.  From the beginning of the story, in my opinion, the reader sees Georg in a better light. 

First, though it is not explicitly stated, it appears that Ulrich von Gradwitz is wealthy. Georg's family is described a petty landowners.  So, right from the beginning, there is a sense that Georg's family is the underdog.  Who does not like an underdog story? 

Second, the text says that the land was not very good. So, the reader naturally asks why would a rich family want land, land that they do need?  Why not give it to those in need or allow them to use it.  The von Gradwitz family comes off negatively. Here is what the text says:

The forest lands of Gradwitz were of wide extent and well stocked with game; the narrow strip of precipitous woodland that lay on its outskirt was not remarkable for the game it harbored or the shooting it afforded, but it was the most jealously guarded of all its owner’s territorial possessions.

Third, Ulrich comes off as angry.  He wants to kill Georg and he is scouting for him.

If only on this wild night, in this dark, lone spot, he might come across Georg Znaeym, man to man, with none to witness—that was the wish that was upper- most in his thoughts.

In light of these points, it is hard not to sympathize with Georg. 

Since this is a personal opinion or reaction question, I'll feel free to state my opinion or reaction, which is that I sympathize with Ulrich von Gradwitz. There are basically three reasons for why I sympathize with Ulrich: story structure; legal possession of the land; first with change of heart.
Saki has structured the story to point our sympathies toward Ulrich. He is the character through whom the story is "focalized." This means that Ulrich is the character through whom the limited third-person narrator chooses to focus our attention on events, both past and present, and on feelings and inner thoughts. This pointing of our focus on and through Ulrich occurs first at the outset of the story when Ulrich, "a man," is introduced as the afflicted protagonist of the story.

[A] man [Ulrich] stood one winter night watching and listening, as though he waited for some beast of the woods to come within the range of his vision, and, later, of his rifle.

Later, Saki includes Georg's thoughts and feelings in the focalization, in how the narrative focuses the readers attention, but the central focalization and primed sympathy is always through Ulrich. One example of how Saki later includes Georg in the focusing narrative occurs during the early moments after they are trapped by the beech branch:

Both men [Ulrich and Georg] spoke with the [feeling of] bitterness of possible defeat before them, for each knew [in his mind] that it might be long before his men would seek him out or find him....

Additionally, the text draws me to sympathize with Ulrich because Saki states clearly that, since the time of the "Courts" adjudicating legal ownership of the land, the land has legally belonged to the Gradwitz family. There is also a strong suggestion that the land had always previously belonged legally to them but that, because of its remoteness, it had been taken possession of by the Znaeyms, possibly in hopes that their possession of "the narrow strip of precipitous woodland that lay on its outskirt..." wouldn't be disputed (or even noticed).

Finally, when Ulrich and Georg are trapped together beneath the enormous beech branch (like the Carpathian beech branch pictured at the hyperlink), Ulrich is the first to offer aid to the other--although his gesture of goodwill and offer of aid are rejected--and the first to feel inner movements of compassion and regret toward his now suffering lifelong enemy.

An idea was slowly forming ... that gained strength every time that [Ulrich] looked across at the man [Georg] who was fighting so grimly against pain and exhaustion.
      "Neighbour," [Ulrich] said presently, "do as you please if your men come first. ... But as for me, I've changed my mind. If my men are the first to come you shall be the first to be helped.... We have quarrelled like devils all our lives ... I've come to think we've been rather fools; .... Neighbour, if you will help me to bury the old quarrel I - I will ask you to be my friend." 

[As a note, Carpathian Primal Beech Forests are now protected by UNESCO-World Heritage Sites.]

Ulrich and George hate each other for impersonal reasons that have nothing to do with them as individuals. Their family history of feuding insists that they feel animosity towards each other, though they have no specific gripe with the other man. The family feud is over land, specifically issues of access and ownership. Relevant issues around poaching and trespassing seemed to have exacerbated the bad feelings between the two families.

In the moments before the howls of the wolves are heard in the distance, portending the deaths of Ulrich and George, the men forgive each other and decide to be neighborly and gracious from that moment forward. The pointlessness of their mutual disregard enhances the poignancy of this moment of their mutual forgiveness, which also becomes pointless when they, and the reader, realize that their deaths are near. After all, neither family will be aware that the men had come to peace, so the feud will inevitably carry on.

The reason that these two men hate each other goes back pretty far in time.  It goes back to the time when the grandfathers of the two men were in control of the lands that Ulrich and Georg now control.  It seems that von Gradwitz's grandfather managed to use the courts to get this land away from Znaeym's grandfather.

Here is a quote that shows this:

A famous law suit, in the days of his grandfather, had wrested it from the illegal possession of a neighbouring family of petty landowners; the dispossessed party had never acquiesced in the judgment of the Courts, and a long series of poaching affrays and similar scandals had embittered the relationships between the families for three generations.

Because of this ancient feud, the two men hate each other.  This is why they are out patrolling on the night that the storm hits.  Therefore, it is that old lawsuit that is going to (it seems) get them both killed.

Ulrich von Gradwitz and Georg Znaeym are enemies because of a long-standing feud between their families regarding the ownership and property rights to a "narrow strip" of woodland owned by the Gradwitzes. The men's grandfathers had been involved in a lawsuit over this land, with the Gradwitzes being granted legal ownership of the land which had previously been inhabited by the Znaeyms. For three generations the dispossessed Znaeym family has never been willing to accept the court's ruling against them and has continued to poach animals on the land.

Now, Ulrich and Georg have inherited the quarrel from their grandfathers and fathers, with Georg continuing to hunt illegally on Ulrich's land. The narrator suggests that the ill will between the two men might have simply diminished on its own over time except that the family feud has become a deeply "personal" one. As children, Ulrich and Georg "had thirsted for one another's blood" and continued to wish each other misfortune as they grew into adults.

The narrator says that the land is "not remarkable" either for the animals who live there or the "shooting it afforded." Thus, it seems that the two men are enemies as a matter of principle rather than anything else. Ulrich jealously guards the land, not because it is of great value or significance but, rather, because he simply does not want Georg to be on that land. Georg hunts on that land, not because he cannot kill similar animals elsewhere but because he seems to want to prove a point.

In H.H. Munro's short story "The Interlopers," the two antagonists are inheritors of a generations-old feud over land. Specifically, Munro, writing under the pseudonym "Saki," places his story in a strip of forested land along the Carpathian mountains that is rich with game. Two neighboring clans or families, the Gradwitz (Ulrich's family) and the Znaeym (Georg's family) have feuded for years over proper ownership and exploitation of this land. Early in his story, Munro references "a famous law suit" between the two families over ownership of the land in which Ulrich's family prevailed. As "The Interlopers" begins, Ulrich von Gradwitz is patrolling this land, watching for any members of the Znaeym family who may be poaching game. This is a blood feud, and violence is an integral part of the relationship between the two groups.

As readers of "The Interlopers" know, the two reigning patriarchs of their respective clans are pinned to the forest floor by a large tree that falls on them and that, during their ordeal, they agree to put aside the feud and make peace. Their plans for a cordial relationship, however, are dashed when they are presumed eaten by a pack of wolves.

Besides the obvious reason that underlies the feud between Ulrich von Gradwitz and Georg Znaeym, which the other response eloquently describes for you, one could argue that the real reason for their feud goes deeper than a land dispute.

Both men are the patriarchs of their families, and both men feel responsible as the leader of their clans to make the right choices in the families’ best interests. For each, this means claiming the disputed tract of land for his family. Is this land really any better than a similar tract that the Znaeyms’ could obtain? Do the Gradwitz' honestly need the small patch of land? The answer to both of these questions is a resounding no.

Therefore, it becomes clear that the land is not what is most important here. Rather, each man wants to demonstrate his prowess and superiority over the other, proving to himself and his family that he “wins.” This dispute is more about male ego and dominance than the land dispute they use as an excuse to feud, which is why Saki’s ending is darkly humorous. In the end, it takes death for these relentless, misguided men to finally quash their quarrel.

In Saki's short story, "The Interlopers," the feud over the disputed land continues because, although the grandfather of Ulrich von Gradwitz had won the lawsuit, the other family, the Znaeyms, "had not acquiesced in the disposition of the lawsuit."  As a consequence, enmity has developed between the families, and lasted for generations; von Gradwitz constantly seeks to catch the "poacher" on his land. So, in the exposition, von Gradwitz patrols the thick forest in search of his mortal enemy.

Of course, the double entendre of the story's title becomes evident when nature becomes an interloper in the form of the storm that knocks down the branches trapping von Gradwitz and the interloper upon his land, Znaeym, as well the appearance of the final interlopers, the wolves.

In this story, these two men do not get along because of a dispute about land.  In the time that Ulrich von Gradwitz's grandfather was lord of their estates, there was a court case between the two families.  They were arguing over who a certain piece of land belonged to.  The Gradwitz family won the lawsuit and that's when the feud started.

The story says Ulrich is out hunting for "a human enemy."  This means that he is patrolling in search of Znaeym or his men -- people from the "enemy" family trespassing onto his land.

In the opening paragraph, Ulrich is patrolling his lands while looking for Georg. Essentially, he is hunting Georg as if he (Georg) were a "beast." Both men are so caught up in their feud that they treat one another like animals to be hunted.

Note that Ulrich most jealously guards this one piece of land even though it is "not remarkable for the game it harbored." The practical problem, from Ulrich's perspective, is that Georg is poaching on this piece of land, even though there is not much to poach in the first place. This shows how stubborn Ulrich is in perpetuating a feud over a relatively useless piece of land.

The fact that Georg has never accepted the ruling of the courts shows his stubborn attitude in perpetuating their feud.

The neighbor feud had grown into a personal one since Ulrich had come to be head of his family . . . 

This statement illustrates how the feud has ceased to be based upon a logical or understandable disagreement. It is now simply based upon an "inherited" malice between the families. In other words, they have inherited and sustained a mutual hate. They no longer fight over the land for practical reasons.

When the two men are trapped under the tree, they continue to be obstinate until Ulrich makes the first gesture of kindness. But, just before the tree falls, there is at least a hint of civility between the two men:

But a man who has been brought up under the code of a restraining civilization cannot easily nerve himself to shoot down his neighbor in cold blood and without a word spoken, except for an offense against his hearth and honor.

This momentary restraint is a very subtle foreshadowing of the eventual reconciliation. It suggests that, however stubborn both men have become, there still remains a faint notion of humanity and possible reconciliation.

In the beginning, and for most of the story, Ulrich and Georg are both stubborn and vindictive. They have inherited a quarrel that is not really theirs. Yes, technically Ulrich owns the land and Georg is therefore trespassing. But to take this argument over an insignificant piece of land to the point of wishing death upon one's opponent is quite ridiculous. It shows how Ulrich and Georg both have reactionary attitudes. In other words, both men are set in their ways and refuse to reconsider things. This is especially evident in how they accept, embrace, and perpetuate the feud they have inherited. 

Their attitudes do change, however. When they find themselves with something in common, the opportunity for change presents itself. While both men are fighting for their lives, trapped under the tree, they are initially stuck in their reactionary ways. But after an initial gesture by Ulrich and momentary resistance from Georg, a friendship begins. They end their allegiance to the feud and they become united in trying to survive. The shift is complete, going from narrow-minded to open-minded, from blindly following tradition to embracing the new. 

This is a great question. Both characters are so similar that it is difficult to say who is more likable from the author's point of view.  However, if I had to choose one, I would say that Georg is the more likable for one important reason. 

As Saki is giving the history of the feud, he mentions that Georg's family were petty landowners.  The implication is that they were not wealthy or powerful.  On the other hand, the other implication is that Ulrich's family was wealthy.  According to the text they took control of the land. Here is what the text says:

This picture is confirmed at the end of the story, because Ulrich lives in a castle, whereas Georg's family presumably lives in a house. The text says that they had a roof. So, from this perspective, Georg is the underdog, and who does not want the underdog to win?