What qualities does each boy have individually (Gene and Finny) that explain why he is the leader or follower?
There are so many contrasting qualities. I would have to say that, above all, Finney is more self-actualized than Gene. That is a term either coined or created by Maslow, a psychologist you may not have studied about yet. The idea is that a self-actualized person is able to be his own "best self," true to his character, not having to pretend to be something he is not, and able to use all of his own abilities to the fullest. That describes Finney pretty well, doesn't it? Gene lacks the courage to be himself, and he both draws upon and resents Finney's self-actualization, leading to the conflicts that we see in the story. Finney is also courageous, physically and mentally, while Gene is cowardly, and Finney is honest and forthright, while Gene is dishonest and somewhat sneaky. All of these qualities make it inevitable that Finney is a leader and Gene is a follower.