I reckon GT5 will sell more lifetime.
This is the first proper GT this generation. There is so much pent-up desire for this game's release, so it'll have a monstrous launch (not saying it'll launch larger than Reach, idk about that). Then the classic Gran Turismo legs will kick in.
Including downloads off the store, Prologue isn't far off 5 million copies... and that was just a snazzy demo!
Believe.