With whom did you sympathize, Ulrich or Georg, neither or both? Why?
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.]