I have to second what tearex said about convertibles in Texas
I had a convertible for the year I lived in Texas, and there were precious few times when I would have wanted to drive with the top down. Even forgetting for the moment the skin damage you will suffer from the sun (melanoma anyone?) the weather just isn't ideal for open air driving. There will be a short time of the day, every so often, when the weather will be comfortable, but the vast majority of the time it will be way too hot or way too cold or raining. Then there is the 500lbs of ballast you will be carrying around 100% of the time due to the weight of the top and chassis reinforcement. All in all an M3 convertible won't even go as fast or handle as well as a stock 335i, and the rear seat is all but useless.
If you must have a convertible save yourself a bunch of money and get a Mustang gt convertible with track package. It weighs about the same as an M3 coupe, has the same power, and will save you $30,000.