Go for the Sport. The Roadrunner is actually kinda cool. It's not pretty, it's not fast (in stock form) but being so quirky and unconventional has some pluses. My guess is that if you have any aspirations of making a performance car out of it, you'll have an up hill battle with that big machine and it's really symbolic of a sad time in the world of Muscle Cars... just my opinion.
This sounds fair for a running car with solid exterior panels. While I will readily admit that the '73+ Dodges are not the prettiest of the A-Bodies, they look pretty darned good in person. Plus, the Duster/Demon/Dart Sport has legitimate and justified street cred and you'll never get people making odd faces as they walk by, asking why you put money into it...for whatever that's worth.
Whichever way you go, good luck and keep us posted!
Do you think you would keep the Dart /6 powered?