GT5 will trounce MK. I'm not saying that MK will do poorly, it will do very good (Several million sold). But I just don't think it can compare to the insane sim that is GT. GT5 will sell a ton of consoles as the defacto must have next gen game, and will end up 1-2 million ahead of MK wii in world wide sales.
Prologue will be a good clue as to how big GT5 will be.
My pokemon brings all the nerds to the yard. And they're like, "You wanna trade cards?" Damn right, I wanna trade cards. I'll trade this, but not my charizard.