I just picked up a 2017 leftover c63 convertible yesterday. I test drove the M4, Cadillac ATS-v and the c63. All of them, in my opinion, would have been a good choice. However, when my wife got wind of it the car went from hard top to convertible immediately removing the Cadillac from consideration. I strongly considered the BMW but the salesperson was really bad and I ended up leaving and going for a second test drive in the C63. In the end I’d have been happy with either. What weighed into the decision was my long heritage with MB and finally they got to a price that made it really attractive. I bought the car in the NE where convertibles don’t seem to sell well, as evidenced by it being a left over, and plan on driving to my second home in Florida a few months from now where I think it will be better used.
After I get familiar with the car I plan on seeing what mods, tuning, etc, are available.