Leper's evasion was psychological while Finny's was physical.
Neither boy could really help what happened to him, though, because it was neither one of their faults. Leper didn't plan to have a nervous, psychological break down once he got to basic training; he just wasn't prepared for anything like war in his life. Finny could have met any physical challenge that the war threw at him before his leg was shattered in the tree incident; but his broken leg that kept him from the war wasn't his fault, either. In the end, Finny and Leper were two totally different boys on opposite ends of the spectrum. Finny was an athlete and courageous at heart; Leper, on the other hand, was sensitive and strong in academics than in bravery. This is why they would have different experiences when facing the draft or enlistment.