Gotta be honest I thought Forza 3 already looked better than GT5 did for the most part. GT5 had it beat with some of its premiums but as a whole Forza 3 was the better looking game.
And Forza 4 utterly blows it out of the water honestly. I never got the GT5 hype. Sure it IS the most realistic sim out there but I just don't see it as on par with Forza both graphically and gameplay wise.
But I can understand the love for Gran Turismo, it just seems really dated to me.