In my opinion, the ps3 version just looks stunningly better than the 360 version. The ps3 version has better vivid colors, more detail on backgrounds etc., while the 360 version seems to lack all of this. the 360 version also seems to have a problem with jaggy edges that is not present or noticable on the ps3 version.
http://g4tv.com/thefeed/blog/post/703052/Final-Fantasy-XIII-Comparison-Video.html#comments