I've played the demos for both Fifa and PES.
I've been a fan of FIFA since the early 90's, but I think I'm moving to the dark side this time around.
PES just feels more natural. You are prone to more elements of human error when playing the game. Every pass is not perfect, every tackle is not executed, and it doesn't feel as if your players are skidding on grease.
I think I'm making the switch.