I've yet to play PES 2011 so I won't comment on your feelings of that game but, if the AI is worse than that of FIFA (10 atleast) which I've played more than enough of to know it is actually beyond terrible.
Have you ever played clubs with the AI on your team? It's set to world class and they can't do ANYTHING right, the main problem with fifa's defenders and fifa in general is how many goals are the exact same, cut backs that keepers/defenders can't ever stop and lobs that consistently go in because the keepers rush off their lines every single time.
FIFA 10 is easily one of the worst football games I have ever played, and that is personally down to the god awful AI and the blatant scripting that goes on during online games, it is unreal.
I've conceded so many goals in the 45 minute + 90th minutes (that seem to last just long enough for them to score) because my defence just seperates, nothing I can ever do about it, the fact that the majority of goals also come from one maybe 2 passes, you never get any time on the ball and pressure/power rule the game, it's just a bad game. Also the defenders always go towards the ball and never mark the guy making an obvious run, even when they run RIGHT infront of them.
Also the freekicks that keepers cannot save, ever. Hold shoot for two bars and push forward, straight over the wall and in, everytime. That and the CPU always score them against you in the exact same fashion, always.
I don't have a problem with you not liking PES 2011 but please don't act like FIFA has superior AI, even if it's the same AI from PES 2010 it's still much better than FIFA's. I've played too much of FIFA 10 to know just how many problems that game has, and I still can't believe the scores it got to this day.
I still have hope for FIFA 11 aswell, even after all that, I'll be buying both games.
(I also believe FIFA 09 is a better game than PES 2009, I have no idea how they got FIFA 10 so wrong.)