Well sev, if you can afford it buy both...the Bentley for those "going to a big event" days and the M6 for taking a "fast girl" out for some fun...
Although truthfully, most converts are heavier than the coupe versions due to the structural rigidity that is lost from cuttin gthe top off, so engineers have to stiffing the chassis. And it gets heavier with hardtop convertibles---those cars that have a coupe like profile, but their"hard" top folds into the trunk.Example are the Mercedes Benz SL550, SLK, Volvo C70, VW Eos, Lexus IS350, BMW 335i/ M3 hardtop convertible. Cloth fabric convertibles are easier to rip open or cut cut with a knife, and jealous people sometimes do that to other people's cars. Examples of these are Jaguar XK, Porsche 911 Turbo cabriolet (convertible).