I DO think Final Fantasy XIII would have been a different game as a PS3 exclusive. However, Square-Enix should shoulder the blame in what's happened to this franchise, not the 360 or its fanbase. Even if no Xbox fans wanted this version, it was SE who put out a shoddy port and performed countless delay after another (although I'm an advocate for "release it when it's done").
Yes, I'm all for exclusives, but XIII could have done better despite the inclusion of another console. If anything, XIII-2 should be proof enough of that.