Differences: Gene is more scholarly (Gene is near the top of his class, Phineas average in his studies) , Finny is (Phineas) the better athlete, Finny is more self-confident (able to step outside convention and challenge authority)
Similarities: there are not a great deal of similiarties between the two teenagers. A similarity lies only in the fact that Gene assumes that his "friend" is as envious of his scholarly abilities as much as he is envious of Finny's athletic prowess. Of course, they have their ages and school in common, too. But Gene is severly lacking in self confidence and maturity. Finny may lack maturity as well, but his is a benevolent immaturity, while Gene's is decidely malevolent.
"As Ronald Weber writes in an article from Studies in Short Fiction, "It is Phineas's innocence that Gene cannot endure. As long as he can believe Phineas shares his enmity, he can find relief, but with this assurance gone, he stands condemned before himself and must strike out against his tormentor."