I believe the PS3 version will be somewhat compromised. Think about it. PS3 has Blueray while XBOX has DVD. I bet they can't fit FFXIII on a single DVD, and if that is the case then something is bound to be dropped at some discs. Other than that I don't see any reason to think the Xbox version will be compromised. The PS3 cpu may have better performance potential, but, as I see it, developers haven't exactly managed to utilize it much more than what you would expect from a 360. And the gpu on the 360 is even better than the PS3 gpu. That the PS3 got better graphics than 360 is in most cases wish thinking or myths (eye of the beholder?). But in the end the PS3 has more potential 'cause of it's cpu.
I think XII was good concerning voice acting. X was quite horrible in many cases; XII was a huge improvement imo. But agree that many animes, games etch. originally developed with Japanese in mind, doesn't sound as good in English. That would be the case with any dub. The original language - the intended language - always gives the best impression.
And, yeah, subs are no problem.