About the environments, I have yet to see a Forza track match up with the London track from GT5. So in that regard GuestLV is not correct. Also about the damage. Compare the impreza from GT5 demo with everything that has been shown from forza and tell me again that forza3 (at this time) has better damage.
From a technical perspective: the shadow in GT5 (p) has artifacts but, also it's more complex. So if they turn it off, it would look 'better' to some. Others prefer the complex, artifacted shadow.
Also with the AA. I play in full HD. For me 1280*1080 2*AA will be more important than 1280*720 4*AA. But an xbox fan which only has a 720p set will tell you both games are equal in the resolution department. To him, they really are. He will not use it anyway.
Same with cars. Let's say GT5 cars are 11.000.000 polygons with 3.000.000 polygons interior. And forza only 10.000.000, including interior. The forza fan would say: well I never drive with interior view, plus I don't see my car, because I drive with bumper view, and I'm always number one so the car models don't matter to me. The environments are what really matters and Forza IMO really excels there!
So that is why we will never know the game which has the best engine and graphics, because the anti-fans will try to talk it down.