Carl on 08 July 2009
Main Series games. (rounded up)
Halo: CE - 6.4Million
Halo 2 - 8.5Million
Halo 3 - 9.9Million
Halo Series - 24.8Million
Gran Turismo - 10.9Million
Gran Turismo 2 - 9.4Million
Gran Turismo 3: A-Spec - 14.9Million (this already is higher than Halos total with 35.2m)
Gran Turismo 4 - 9.7Million
Gran Turismo Series - 44.9Million
I think its safe to say which is "stronger".
And not to forget, Gran Turismo has wordwide selling power.
