Honest opinion. Ok fine.
First I'll point out that I've played the forza demo (own it), GTP (own it), Gt4 bmw, GT4 nurb demo (with that uber fast skyline).
Don't get me wrong, I think Gt4 is gonna RULE, but Forza also stands on it's own, it has features that GT4 has, that I wish it did. Granted, it doesn't look as nice, but it sure as hell feels fun to play. I just feel that GT really hasn't innovated much since GT1, which is just abit of a shame. What's there is pretty polished though. The cars in forza feel wilder, more powerful, when you hammer the throttle, the wheels spin like mad, causing the car to shake wildly etc, GT has allways felt abit too tame.
Gt also misses things out, for example I find it comical that on the drag races you can only have ONE car on at a time, you can only have a very limited number of some cars on tracks, no working headlights, no damage, poor sounds, hell after all these years they should be UBER MASTERS at this stuff.
And yes, GT does have this certain appeal to it.
Skidmarks?