Gran turismo 3 = 95 metacritic
Gran Turismo 4 = 89 Metacritic
Forza = 92 Metacritic
Forza 2 = 90 metacritic
They are pretty much reviewed exactly the same (add them together GT wins by 2 points). However, Gran Turismo has been around a lot longer than Forza so has been able to aquire a much larger fan base. That is pretty much all it boils down to. The games are very comparable as far as graphics, features, etc.