I was just blown away when I played Forza 3, maybe I'm missing something but GT5 is on such a lower level. Forza is SO much more visceral and realistic, you can actually feel the physics of the car as it turns and accelerates and watch the body flex and roll with the suspension. And don't even get me started on the sounds, get a '70 Charger in GT5 with an exhaust upgrade and it sounds like a freakin' weedwacker on nitromethane. Speaking of upgrades the whole tuning system in Forza is also light-years ahead, you can actually tune specific parts of the engine (cams, intake, compression, etc) or even swap engines entirely; GT5 just has the lame "Engine Level 1, 2, 3" and "Turbo Level 1, 2, 3" type system.