This GT5 vs Forza Series thing is hilarious and desperate. Lets look at the most accurate, up-to-date numbers we have available. We have GT numbers up to September from PD and we have Forza numbers from RIGHT NOW on VGChartz.
GT5 + GT5: Prologue - 14.5 Million units.
http://www.polyphony.co.jp/english/list.html
Forza 2, Forza 3, Forza 4, Forza Horizon - 12.4 Million units.
http://www.vgchartz.com/gamedb/?name=forza&publisher=&platform=X360&genre=&minSales=0&results=200
1 game and a glorified Demo vs. 3 main games and a spinoff. Just face it. Forza is nowhere near the level Gran Turismo is. Never will be. Microsoft can release a game Yearly. They can bundle it every Holiday. Its simply not desired by the public half as much as Gran Turismo. Just give it up already.
