I don't know why people say they can't be cross-shopped. I was looking at the GTR when I had my M3 on order. Both cars make sacrifices in different areas and it's up to you to weigh the pros and cons of each car. Yes it's a good 20k more, but it's not too much of a stretch. The big thing for me was being able to drive a manual. I'm just not ready for a dual clutch system yet. I don't have a daily commute so daily drivability wasn't really a factor. I also don't like the interior quality or video game-like center console/nav system and I prefer high revving N/A RWD cars overall.
What piqued my interest in the GTR was the insane performance bargain it was, the menacing looks, and all the technology around the car. But in the end I didn't want that for an extra 20G and the M3 was just a better car for me overall.
(1:40 @ Laguna Seca, 2:03 @ Thunderhill, 1:52 @ Infineon)