I got a PS3 because I wanted to play The Last of Us and things like Journey/Uncharted as I'd previously been a 360 owner and hadn't played them.
Now that I've finished TLoU, Journey and thought Uncharted was sh*te (PS fanboys were on drugs saying it was better than Tomb Raider), I am stuck with the PS3. So I preordered GTA for PS3.
However.......given the choice I'd get another 360 for GTA and most other games. Games just seem consistently smoother on 360. Also I never watch Blu Ray movies as I rent digitally these days.
Oh and yeah I prefer Forza to GT due to their philosophy of 60fps. I played that GT6 demo/academy thing a couple of months back and the frame rate drops were an absolutely joke. Just like GT5.