I never had a convertible, nor had I driven an M3 let alone the convertible before getting it and I don't regret anything one bit. I really couldn't care less about the weight difference. The car is fast enough for me and a 400 pound difference isn't going to outweigh the fact that I can put the top down.
Having done an ED with the car during summer was truly one of the most amazing trips I've been on. The coupe can't compare to driving with the top down on a sunny day and the benefits of being able to hear the exhaust and engine that much more. With that said, convertibles just aren't for everyone either. I spend my time now awaiting warmer weather for the day the top can go back down