The funny thing is, I think Sybok could work out okay as a character - just assume Spock has a half brother who was an adult (or teen) when Spock was born, but doesn't accept all the Vulcan horsepucky and gets more or less disowned as a result. That's an interesting factor to add to Spock's backstory.
Re Sylar vs Spock - I feel like I sorta know how Spock's childhood was, but Sylar's is a mystery. Was he a normal kid with a somewhat flaky mom till the "bad genes" kicked in? Or was he screwed up by his parents, a la Elle, possibly because they guessed that powers would emerge? Mom didn't seem to know specifics, but what about his till-now unseen dad, who was apparently someone Sylar was either desperate for the approval of, or deeply resented, or both?