I sold my 2002 M3 this past summer after having owned it for a little over two years. The M3 would be faster than the GTO, has better brakes, and would undoubtedly be faster on a road course. Quality wise, the M3 had better paint and the fender and door lines were better. Interior wise, I don't see where the M3 had an advantage as I am most impressed with the GTO's interior. However, it was much easier to get to the rear seat in the M3.
While I loved the way the M3 engine revved, it is hard to beat a good ol' American V8. Where the M3 may be faster in a straight line contest, the torque of the LS1 makes it more enjoyable in day to day driving. Not to mention that the exhaust soundtrack is infinitely better on the GTO. I replaced the M3 muffler with an aftermarket exhaust in a quest to get rid of the raspy tone with no luck. I am not even thinking about changing the GTO's muffler as the sound is a perfect blend of classic V8 sound combined with the unbeatable soundtrack on the overrun when you back off on the throtle.
I fixed the lack of exterior temp gauge by installing an aftermarket mirror. However, my M3, which was fairly basic at a $50K plus sticker, did not have power seats. While the M3 seat was very comfortable and had many manual adjustments, it was MANUAL! Considering that I paid a little over 25K for the GTO, I could have two of them for the price of the M3.
Which brings up another issue. I have had two Corvette C5's and loved both of them. How can Corvette justify their prices compared to what you get with the GTO. Not to mention, the GTO has to be shipped from Austrailia!!