PS3 version, easily. Batman: Arkham Asylum has shown just how prevalent multiplatform ps3/360 gamers are, as it was a Western game that sold significantly better on ps3 in NA solely due to exclusive in-game content. There are seemingly hundreds of thousands of gamers that would normally buy something for 360 unless there is a good reason to do otherwise.
FFXIII will also sell better on ps3, having the advantage of being on one disc as opposed to 3-4, and supposedly having the advantage of looking slightly better, if IGN is to be believed. Much like the Joker content, this will give ps3 the advantage with multiplatform gamers.
And that's not even taking into account probable demographics (ie, FF fans more likely to own ps3s than 360s) and the fact that FFXIII was ps3 exclusive for years.