Maybe the 360 version of FFXIII will be downgraded (my Thoughts)

[Vagabund]

They won't downgrade it because they don't care about the performance (-> Last Remnant )

Joking aside: I believe the difference will be minor, like with almost every multi plat. Maybe a slight advantage to the PS3, since it's the lead platform, but the game will look and sound great on both consoles.