SE has had a bad enough rep on the 360 that I would think they would be extra careful to avoid slowdowns and tears.
I'm pretty positive that the PS3 version will look better solely because its the console that the game is being developed for and its being ported. I think PS3 owners have a lot of experience when it comes to that.But I think that both versions will push its system to its limits.
Square said that there shouldn't be as many disks as one would think because FFXIII has more cut scenes using the in-game engine. So I suspect 2, 3 at the most. Square also said that they are putting great effort into keeping the quality the same on both consoles.
Wow the MP system is just called the cost system now you don't have to freak out because its no longer called MP.