We rented a Chrysler Sebring Convertible a few years ago from Alamo at MCO.
I had decided against one because of the price, and reserved an intermediate size car, but we were offered an upgrade for quite a bit less when we got to Orlando. This seems to be normal practice at most car hire companies so it might be worth waiting to see whats on offer when you get there, unless you've got you heart set on a convertible.
For us (2 adults and 2 children), the novelty soon wore off, and we find a nice roomy car with good air conditioning works better for us.