Of Bill and Henry, who is the optimist and who is the pessimist? How do their personality traits relate to their survival?

One would have to consider Henry as the more optimistic of the two sled drivers. He's a much more rational, thoughtful individual, who takes time to think before acting. He seems completely unfazed by life in the wild and prefers to use his brain to get himself out of a jam. This serves him well, as he's able to use his capacity for logic and clear thinking to outsmart the wolves and save himself from an almost certain death.
Bill really couldn't be more different. A pessimist and constant worrier, he simply lacks the requisite skills to survive in such a harsh, unforgiving environment. And so it proves. Instead of using his brain to ward off the vicious pack of bloodthirsty wolves, he impulsively grabs his gun and starts shooting at them. But this is a big mistake, because the wolves know what a gun is and so are able to run off at the sight of one. Bill foolishly and desperately runs after the pack, determined to shoot them dead. But his impetuosity costs him his life as the wolves attack and kill him.