The PS3 and 360 versions of FFXIII have been compared extensively since the 360 version launched back in March.
The main difference is in the CG scenes where the video had to be compressed more in order to fit in the DVDs of the 360 as opposed to the higher capacity blu-ray of the PS3. Additionally the game had been developed for the PS3 over a number of years, and was ported in a relatively short time to the 360.
360 owners can take comfort in the fact that this is probably the only cross platform game that looks better on the PS3; in almost every other case the opposite is true. This is partly thanks to the 360's much more powerful GPU, and partly thanks to the much simpler architecture and far superior dev tools: if you spend the same effort/money building a PS3 version and a 360 version, the 360 will always end up looking much better. In the case of FFXIII Square Enix simply spent a lot more time optimising the PS3 version.
By the way, I own both consoles, and I actually prefer using the PS3 as it's much more quier so hopefully I won't be accused of being a 360 fanboy.