Well, as I said, I'm going to take a look at GT6 because it's on a system I already own, and because it looks like they may have addressed some issues.

I played GT5 enough to know I was rather disappointed with the lack of progress however. I bought the game, and I put enough time into it to know that I simply wasn't impressed. Neither physics engine is perfect, but for use with a controller at least, the feeling of cars on track at speed felt more real to me in Forza than GT5. Since I have legitimate seat time to compare to, I do have some experience here.

Combined with real consequences from damage, or over-revving, etc, and better computer car AI, it felt more real overall.