We will have to wait and see for GT5, although it will be hyped massively and i expect a pricecut either before or around the release.
Halo 3 should be at 10 million before GT5 is released.
Anyway... Gran Turismo is WORLD popular... Halo is really popular only in America, and seeing that GT5:Prologue just hit 3 million at retail... its gonna be huge when the real game is released.
My money on this goes to GT.