Weight is One of two main factors, the second is tortional rigidity. The conv has ALLOT more chassis flex then the coupe. This is turn means that the conv will not be able to handle as well as the coupe. Consiquently the sedan has even a higher regidity then the coupe, but not much. Once this when you buy a BMW never get the split folding rear seats, they represent a major loss in chassis regitity. Both coupe, conv and sedan, if they are a 5speed manual are geared the same.