I personnaly have no interest in convertibles but I do like the look of the fastbacks.
Yes you have a pony car but the Challengers and Camaros are also pony cars and some of them where also muscle cars. A 65 Mustang Shelby GT350R would be a true muscle car and still a pony while a basic 6 cylinder Challenger would not a be a muscle car

. Still the basic Challenger coupe would have more value than a Mustang coupe. Is it because they never offered a fastback Challenger? I love the mopar muscle or pony cars but I will never be interested to pay that extra money to own one.
Here is where I got my source:
http://www.musclecarclub.com/musclecars/general/musclecars-definition.shtml