OK LOL, heres my input.
1. The GTR is a much safer car comparing to the RWD in the M5. AWD to me almost feels boring even though you may be faster off the line.
2. Exterior the M5 is a beauty, although the GTR may turn more heads then the M5. Definitely more dudes.
3. The M5 is not for you if your gonna be afraid to take it over the 100mph mark or 3rd gear. This is where the m5 really performs. If you just want to sprint off the line all day then the gtr will be better.
4. Interior of the m5 is 10000x better then the GTRS OBV.
5. Build quality is definitely better in the M5 as others have said.
6. Stock exhaust the M5 wins.
7. The GTR may handle better even though they are very similar in size.