I guess if you've already taken the plunge for next gen then there's no point rebuying another PS3 unless you have money to burn.|
GT6 is by far the better game. It's basically how GT5 should have been (all the bad parts taken out). It has more content and is actually fun to play. But obviously Forza 5 is better graphically.
You would have been better off buying a PS3 now and waiting for more worthwhile games to come out for next gen when they will be cheaper. But as said, its to late for that. You've been had. Might as well stick with it now you £500+ down.