Toefactor on 18 March 2009
To be honest, I don't think it would even be close upon further consideration. The Halo series averages about a 20-25% attach rate. Gran Turismo is nowhere close to that. I mean the highest selling GT game sold about 14.5 million copies on a console with a 120 million user base? (around 8.25% attach rate)
So going back to the OP's scenario of both games being released for consoles with an install base of 50 million Halo 4 would outsell GT5 by millions.
