Actually there is a difference between the two versions, but it'll only affect those people that are crazy over quality. The PS3 is getting completely uncompressed audio and video while the 360 is getting compressed audio and video. Since the 360 version has not really surfaced yet apart from I think being shown at events, no one really knows if the compression is going to be a serious issue or not. It's unlikely it will be a major problem, but who really knows how much SE compressed things. We've only seen the final product of the PS3 so far.

There is also the case that the White Engine, the engine that FFXIII was built on was built for the PS3 and the PS3 version was done first and then ported to the 360. So there are may be unknown issues due to that case. Who really knows.

I'm getting the PS3 version myself because I only own a PS3. Though I also think it'll be the better of the two since the game was developed on the PS3 and typically a game will function better on its native console.