It all has to do with the controller used.
Forza was designed for a hand held controller, and what little "feel" there is, is catered for by the controller.
GT5 does in fact support and was designed specifically for a clutch and manual H-gate tranny - thats why it supports the Logitech G25 so well.
On the US GT5 disk there is an interview with Kaz, and you get to see that no matter what the Forza team clained in all thei rinterviews, they had no true passion fo rthe title - the GT team has passion, and when you hear Kaz say this iteration (GT5) is still only about 15% of what he invisions for the GT Franchise, you know its going to only get better over the years
Then again, I only "play" the races/levels and contests to open cars, tracks and options, then the real enjoyment I get from the GT franchise is hotlapping and open races - not the "set races" so much.
Put R3 tyres on, drive within a realistic speed envelope, and I find GT5 is 10000000x more rewarding than Forza2 - because I get to use the G25 wheel and because when you are done, just kick back and watch the most fanastic replay camera work this side of live TV