GT5 other than the great graphics, wide range of cars that they are proposing and the excitement to their license mode, i dont see any other features yet to compare with Forza.

Forza 3 on the other hand has car damage, custom paint jobs, driving assists, different camera views (interiror view ), better physics not graphics, and an online trade system.

Hard to say which is better since GT5 has not release any additional news on the game therefore, i would say Forza 3 is the best racing game in its genre.