Owned both for the same amount of driving time and I love both cars but it comes down to what you're using the car for.
For reference I put around 15k miles on my 2013 GTR and about 14k miles on my 2018 M4 DCT
DD: the M4 wins, not even close. The soft suspension mode on the GTR is weak, might as well keep it in normal mode. The adjustable settings of steering/suspension/dct/throttle makes the M4 a much more bearable car driving on the streets. The M4 has a lot of sound deadening so you can easily hear music in the car, my GTR felt loud at all times even with the music bumped all the way up. Also the transmission has a lot of "mechanical" sounds that you have to get used to. One thing I'm surprised most people haven't mentioned is the amount of time the GTR takes to warm up the transmission. I've always babied the GTR after cold starts up to the operating transmission temp, which would take atleast 10+ miles. The M4 I feel warms up in half the time, so if you live close to work then the M4 has more fun per miles
FBO mods: Clearly the GTR is faster and responds to mods well but from a roll its not a huge difference. The GTR puts down power better but the M4 can hold its own from a roll. The launch control of the GTR was my favorite part, makes the M4 laughable. At the end of the day the M4 feels very fast overall, no slouch what so ever
Fun factor: I honestly believe the M4 is more fun. the M4 can go sideways as it pleases and you need more skill to drive it at the limit. The GTR has a huge amount of grip and its much easier to drive, but did you really enjoy the driver connection to the car or did you enjoy the speeds that the GTR is capable of without much effort?
Reliability: My GTR never had issues but the transmission was always on my mind. It was a given that at some point I'd have to upgrade the transmission at some point even though DBA's (2012+ GTR) are more reliable. The M4 has a crank hub issue thats always on my mind so regardless of car, both has their own problems. I'd take the GTR's transmission issue over the crank hub though. Cost wise of maintenance the cars are pretty close.
Looks: The GTR has and always will be a head turner. It might not look pretty to everyone but no one can mistake it for another car. Its big and it does not share the body with another model unlike the M4/4 series. The GTR is a massive dick magnet though and people will always try to race you, follow you around and egg you on. If you like the attention then the GTR is for you. The attention does get old over time.
In the end if you daily the car more, you'll love the M4 more.. more space, more enjoyable experience from point A to point B. If you want a toy on the side thats a rocket then the GTR is better because its the king of streets mod for mod. I hate to say it but the M4 is a better refined car, the GTR will always be a very fast toy.
