ok, im a Forza boy!....
Gt4 looks impressive, but what i've allways hated is the countless numbers of skylines, in 450000 "different" versions.. sure the car fan knows the difference, but us normal people dont! they're useless!!! Same with the Subaru's and the Evo's and the Supra's..... bahhh! duh! and you'll eventually tune them into the unknown side of the soundbarrier... so what use does it have with going back and start over on the "same" car???
Forza has less cars yes! but has that customization level that is extreme!! every car has 6 "sides" each capable of packing 100 layers of decals each!! thats 600 decals!! go online, and you'll never find a car thats the exact same as yours!
The aftermarcet parts are about the same, but forza has spoilers and bumbers, that actually change the cars preformance, adding a front bumber adds front downforce, etc....