I really think you can't go wrong with either if you're playing friends. I'm having an absolute blast with both with "sim" people I know.
For regular online matches, it's honestly still pretty hard for me to find people to go up against in PES. I think part of it is because there's so many splintered smaller online communities because you can search for all sorts of different online matches. So it's taking me some time to find matches when I head on, which is not good. On top of that, how the team selection works means you run up against Barcelona or Real Madrid most of the time unless you're in MyClub.
With FIFA, obviously it's easy to find a match. The tactics aren't as deep, and while both games are a bit faster and more frenetic online, FIFA still is a little more hyperactive, which I consider a negative.
So my way to get out of this is, which gameplay style do you prefer? From there, is ease of finding matches important? Will you play myClub or real teams? Do you play friends or randoms? How you answer these should help.
As a personal preference, against friends -- in part because I have more who play FIFA -- I'm choosing FIFA still. For random matches (assuming I don't have to play Real all the time), I like PES more.