Yes Forza has better AI (if you can call it that) and damage models (again, if you can call it that), but there's one reason why Forza doesn't own GT4... GT4 wasn't horribly executed.
And get this... Deepblue may have killed all the threads regarding "look at the pretty graphics", but this thread isn't about "look at the pretty graphics". It's more like "Look how big an advancement over the rest of the franchise this is." I mean hell, to see 18 cars on the track, each with it's own driving style, each one not sounding identical to every other, I think it's worth mentioning that this will be a HUGE improvement for the franchise.