Forza is much better.
Games are ment to enjoy, and though GT4 is very fun, I have got to say that those 24 hour races are a complete turn off. Its just too much for a game. No game, IMO, should go that deep, and involved. I mean 24 hours? Damn....
On the other hand Forza is great with the realisic physics and damage. The game is perfect in almost every way. Gotta love it.