According to IGN, the PS3 version does indeed look better:

http://xbox360.ign.com/articles/106/1061264p1.html

Between disc swapping and graphics, I guess my mind is made up.

They also mentioned they like the battle system and the character growth (Crystarium) system.

However, unlike many 360 to PS3 ports (or vice versa, like Bayonetta), the 360 version runs as smoothly as the PS3 version. The article above mentions that, as does this article:

1UP's RPG Blog : Final Fantasy XIII Goes Cross-Platform, with Aplomb