Fact remains Forza 3 is still better. Not about sales as GT5 has been around a LOT longer then Forza and has a huge international following, which Forza Motorsport would never get immediately as it's an American made game. Japan hates American games.
Game Rankings:
http://www.gamerankings.com/xbox360/948030-forza-motorsport-3/index.html - 92.1% Forza Motorsports 3
http://www.gamerankings.com/xbox360/948030-forza-motorsport-3/index.html - 84.6 GT5
Metacritics.com
http://www.metacritic.com/game/xbox-360/forza-motorsport-3 - 92% - Forza Motorsports 3
http://www.metacritic.com/game/playstation-3/gran-turismo-5 - 84% - GT5
So, it's not just me who believes this. The fact remains the same. When Forza 4 is released it will be even more convincing to critics. The King of Racing will continue into the next generation. Nuff said.