prosthetic leg because you cant or shouldnt kick your opponent. a well trained prosthetic arm can increase your punching power tenfold.
Depends on how much mobility you have on the leg. If you have some mobility still, it may sound crazy but your legs actually weigh a decent amount. Thats more weight to put in the upper body. So lets say your chavez jr who already blows up to 185 at middleweight... You could have him fighting at light welter weight probably.