Imo Forza 2 was fun, but GT5: Prologue is very fun. GT4 graphics were brilliant at the time, don't compare them to Forza 2 on 360. There was a large deal of understeer in GT4 but GT3s physics were brilliant, and to me Forza physics have always been more Arcade-ish than realistic. I'm not saying they are like arcade game's physics, they are just more arcade-ish than realistic. Forza 2 graphics (to me) are good but i think a few people will be able to agree with me that Prologue looks better than real life :D.
The sounds in GT4 were a big let-down and the sound from my Ferrari 355 on Forza impresses me every time but Prologue now sounds like the actual cars, especially the 430 and M3.
In my opinion, GT5 Prologue owns Forza 2 except for customization, but GT never aimed for it, and i don't think that bothers people too much. And of course the lack of cars and tracks in Prologue but come on, it's a demo.