GT5 have fully modeled interier. Forza 3 is using textures for details (thus they aren't fully modeled).
IMO there is no comparasin, GT5 is visualy generation ahead of Forza 3 when it comes to cars and their models.
MY HYPE LIST: 1) Gran Turismo 5; 2) Civilization V; 3) Starcraft II; 4) The Last Guardian; 5) Metal Gear Solid: Rising







