I can't be impartial as I own just the 360, but honestly, I think exclusives wise it's miles better than the PS3... I mean, on 360 we had 2 fully fledged forza games over the duration of GT5's development time. PGR games are fantastic to play, and whilst Bizarre have jumped ship to activision, the series is still alive... On PS3's side there's motorstorm and GT5
However, if you have a G25, I'd go for PS3, as they support the wheel. Microsoft have a fanatec offering, which I hear is good... but it's very costly, and there's no sign of G25 support going to Xbox.
If I were you, I'd check for a second hand xbox, trade-in places are only offering £30ish for xboxes now the slim has come out, so you should be able to pick up an elite on the cheap.