Go the sedan!
Sedan and coupe are identical performance-wise, plus you will have something a little more convenient for the little one and the sedan is a rare sight on the roads (in my area at least).
I understand convertibles have to be heavier, but an extra 200kg than the coupe or sedan? That is a massive deal breaker for me, and I am not convinced on the styling.
BMW E92 M3 6MT, G-Power SKII supercharged, custom exhaust with 100 cell cats and Remus Sport mufflers, Brembo 380mm BBK, BMW Performance steering wheel, LUX H8 angel eyes, CF CSL boot, Hamann CF diffuser, Vorsteiner CF lip, CF grilles, CF gills, CF bonnet vents, BMW Performance stripes, RAC RG63 wheels, Eibach lowering springs, camber modifications.