Extreme either/or polls like this are always treacherous, but this one isn't quite as much, at least for me. Prologue is so darn good that it makes my PC sims seem... plain, downright boring in comparison. I'll play them a bit, but the graphics are really atrocious, the physics and A.I. are only marginally better, and the lack of car variety leaves me cold. I just keep going back to Prologue for my racing fix.
Give me a game which is a quantum leap over Prologue, the real Gran Turismo 5, and if it has race mod and a livery editor, it's entirely possible I'll never look at another series again.