Some of the comments in this thread are beyond bitter. I've been playing GT5 for days and have actually been one of the only ones to post a legitimate, thoughtful comparison between the two, however it didn't surprise me that most glistened over it in favour of these petty screenshot wars and list comparisons.
People saying Prologue was a superior game to Forza 3 honestly have no foothold to stand on. Excluding the car models themselves, Forza 3 is superior to every aspect of Prologue, and this is coming from someone who bought the Prologue bundle with his PS3, a long time GT fan.
Quite simply however I have to retract certain aspects about my previous analysis, I actually now prefer Forza 3s car physics over GT5s. I commend PD for improving them immensely (they were the most disappointing aspect of Prologue for me) but slotting Forza 3 back in this weekend was a breath of fresh air. All assists off, Forza's cars just feel more alive, at times GTs just decide to randomly oversteer, and it's definitely a problem with the game, I've had a close friend comment on it too. With Forza I can manage to wrestle the car into staying where I want it.
That's not to say I think GT5 is a bad game, I'm loving it, but after the time it took to come out, I'm shocked and frankly disappointed that I can have a much better (in my opinion) driving experience playing Forza.