In a broad sense, Buck and Spitz do have some similarities in their abilities as leaders. Both are physically fit alpha males, highly determined to come out on top in any situation. Each dog is effective in getting the other dogs to fall in line, creating a hierarchy that works for the team. Yet such similarities are surface level; it is in their personalities and methods that their differences are apparent.
Spitz’s ruling method are Machiavellian, to say the least. He rules through fear. With no sense of morality, compassion, or bonding with the other dogs, he feels free to wound or kill any time he needs to ensure that he has control. For example, when two new dogs are added to the team, Spitz “proceed[s] to thrash first one and then the other” as a way of greeting them, making one cry out “when Spitz’s sharp teeth scour[s] his flank.” The rest of the team falls in line behind this bully merely to avoid conflict. Yet as with most bullies, Spitz is a coward, too. In one attempt to take Buck down, Spitz waits until the bigger dog is exhausted after running to escape a rabid dog. “He sprang upon Buck, and twice his teeth sank into his unresisting foe and ripped and tore the flesh to the bone.” Spitz knows Buck is a threat, as the only dog who refuses to cower before him.
Once Buck is able to eliminate Spitz, his leadership style—leading by example—is a clear benefit to the team. Even their human masters know this, as Francois says, “An’ now we make good time. No more Spitz, no more trouble, sure.” Rather than bullying and putting the other dogs at risk, “Buck took up the duties of leadership; and where judgement was required, and quick thinking and quick acting, he showed himself the superior even of Spitz.” Buck has the guts and the strength to bring their crabbiest dog, Joe, in line the first night, “a thing Spitz had never succeeded in doing.” Having suffered much himself, Buck also feels compassion for the dedicated dogs on his team, such as Dave, when he is willing to work himself to death. Most of all, Buck takes pride in working hard and makes sure the others do their fair share.
The one time he refuses to pull, allowing the ignorant new owners to beat him nearly to death rather than lead the team forward, Buck is actually trying to save all their lives. The other dogs follow his lead, refusing to move, even though they are beaten. After John Thornton rescues Buck from the harnesses and the rest of the team moves on, we learn that they all break through the ice, to their death. Had the humans accepted the the wisdom of this brave, experienced mixed breed as well as the other sled dogs did, they all would have lived. The only human to truly recognize Buck´s qualities is Thornton, who respects Buck enough to let him be the authority of his own life in the end.