I've got both games and it's easy.
EA do a lot of things well but when it comes to the physics in game having a bit of scripted motion capture and everyone doing tricks like Jay-Jay Okocha might have worked well on the American sports franchises where you score 100 points per minute but with football I have to feel like I am moving every muscle, every move and pass on and off the ball.
I've played both games since FIFA '96 and the old ISS Pro and with FIFA you pick it up and play it, it's FIFA. With PES I feel like snapping the controller in half after 5 games because they've made all the little niches different, the dribbling different, the turning and passing, the AI. But you have more freedom and less scripted moves, I love it after I've got used to it. Most FIFA players just laugh at the silly team names in PES, pick up the game, can't play it and then say 'this is shit'. It's sad because they are two good games in all honesty, but at some point you pick one because you like the way it plays. Or you pick one because your mates play it. Or because it has bells and whistles on it if you are under 14.
In my opinion PES 2011 is a good game, as a complete package probably less than FIFA 11 when you consider the online and all the modes, but from 2008 to 2010 Konami were in danger of falling out of the race with stagnation. They changed in game but nothing else.