The one thing about PES is that people online, people are more likely to tweak with tactics than they are in FIFA. I personally think FIFA has a more simplistic tactic system BUT PES' has more of an influence on how the match plays out.
I've mostly been playing PES Co-op online and enjoy it apart from the fact that Konami will let you start a match 1 vs 1.
To me, just like Madden & 2K, FIFA has enough of the market share to force users to play a more realistic game but they have shied away from angering their primary base. I can't blame them for that, I just wish that more gamers were like us and want a closer representation of what we watch on TV. I know quite a few people who love FIFA but don't really watch the sport unless it's the World Cup.