Definitley FIFA, Having played both i can honestly say PES is awfull. What gets me is that "most" PES fanboys claim PES is the most realistic football game and FIFA is no where near, but as you can see from some of the posts here that is far from true and they are just stuck in that "mine is better than yours" and wont give 5 minutes to FIFA to see they were wrong.
If you buy FIFA you will not regret it, a much more realistic, fun and full game. They have the licences for almost all the leagues (including championship/div1/div2 etc) Proper Team Names, Kits, Logos... a reasonable amount of the stadiums (major teams and national some 20 in total about).
For me the simple fact about the kits logos team names player names etc is a REAL downer.... the game just doesnt feel "right".
Anyway the choice is yours, i recommend you try both but the demo isnt much to go off the FIFA demo was pretty poor to be honest but i knew the game would be brilliant and it was/is, my most played game for sure.... PES i played full game.. its worth a go but wouldnt sway my decision.