I'm going to skip this round as their is no way to compare these two IMHO. How good a driving simulator is Reach? Rubbish. How good a FPS is GT5? Rubbish. This is a comparison that has no criteria to judge on really. Heck GT5 isn't even a traditional video-game really, it's an attempt (as with 1 to 4) to create a simulator in a console environment.
I'll just watch the sparks fly I guess.
Try to be reasonable... its easier than you think...