Athleticism is best described as speed, strength, and agility. A fighter can make themselves a better athlete but that doesn't mean they will become athletic nor does that mean that athleticism beats someone who is not athletic. Athleticism is useless if the fighter does not punch smart.
Well, not entirely useless. It can be effective on its own to even capture a world title. (see: Javier Fortuna) When you get up to a certain elite caliber of competition, though, you do need more than just raw athleticism: preferably a modicum of technicality, or at the very least good timing and placement, and some ring IQ.
Athletic would seem best used to describe someone who is good at an athletic endeavor. Carl Froch is very good at boxing. He is very athletic.
I'm thinking Amir Khan (fast) vs. Chris Algieri (athletic, super athletic... and incredibly hot. I, erm, what?).