I actually got chance to drive a convertible M4 for about 3 weeks, I've got a coupe normally.
Now admittedly, it was a newer production car than mine, so there were some other differences anyway. I'm also more of a Friday commute B-Road warrior than a track guy, take that as you will.
Anyway things I would note when comparing:
1. Vert is noticeably heavier back to back, meaning its more comfy but also sways on bends more and doesn't have the same kick back on acceleration. I mean it will still cream most other cars, and it feels fast. But when I got back in mine you feel it. You can really tell the roof is missing when it's down and you are trying to press on.
2. I actually preferred the seats on the vert, didn't hurt my thighs on longer trips as much.
3. The vert doesn't get as many looks as the coupe with the roof up, and it gets more with the roof down, even more during the roof up/down process. If that matters to you of course.
4. I really missed my carbon roof, it makes the coupe look far more "meaner" in my opinion.
I see you are in London, so I'd suggest the convertible if you are going to be cruising around town or on motorways most of the time. If you expect to have time to gun it on A/B roads or the track, the coupe will get your heart racing faster for sure. Both are great.
Enjoy your new car!
