PS3 is gorgeous. The 360 doesn't even come close to paralleling it's graphics
From personal experience, they're virtually the same. I play my Xbox on HD, and the PS3 I played on was also on HD, and there was almost no difference in graphic quality.
But Sirrus, those are 2 totally different games using totally different engines. :o
I'm pretty sure they could get same/better graphics on Xbox too if they ported that game professionally.
Protip: Graphics don't make the game. Gran Turismo has always been a very boring, mediocre series.