I'm not bothered about FFXIII going multi-format. I'm still planning to buy the PS3 version, because that's the system the game was originally designed for and should therefore be the best version.
I wouldn't be so sure about that. We don't know how long the 360 version has been in development, and I'm willing to be Squeenix aren't going to fire out a sloppy port.
Apart from the 360 version being multi disc (we assume) I doubt there'll be much difference between the two.