I did a similar trip to this in August last year. I had a Luxury Category booked (Alamo) and had a choice of MB C250 or Lincoln MKZ. Sadly I cant comment on the convertible but I have seen Mustangs and Camaros V6 at other locations. I know from renting a camaro SS Cabrio from hertz the boot is tiny and you will not get a suitcase in with the roof up or down. Also if you can use the automated Alamo Check in machine it saves you from the dealings of the desk clerks who can bamboozel you with unwanted add ons.

Just out of curiosity where are you going on your road trip?. I hope New Orleans is on it.