Okay, we can finally get some facts on the table.
http://www.eurogamer.net/articles/digitalfoundry-final-fantasy-xiii-face-off
Here's the essence of it: PS3 version is more impressive than 360 version. 360 version has lowered resolution, and "amateurish" compression on cutscenes, but manages to maintain a higher framerate due to the cuts in picture quality. The conversion could have been much better though, as the 360 version is but a quick port.
My biggest gripe with the 360 version is the characters' hair. It didn't look good on PS3, but the lower resolution makes the grainy part more pronounced.