Which is better, '68 Mustang Coupe or '76 Corvette?
Minor rust? Check the cowl for leaks, check the frame rails, floor pans, shock towers. Minor rust has a bad habit of becoming major once you really get into it. If it has thick undercoating so you can't see the pans well, or the cowl leaks, the Mustang isn't going to be the best choice.
I'd suggest doing the same of the Corvette. Just because the body is fiberglass, the rest isn't and that can mean hidden rust issues.
If both clean and the Stangs rust really is minor, I'd go with the Mustang. I'd also say you find out who rebuilt the motors. A reliable shop, with receipts to prove it, or a back yard shade tree mechanic? There's no guarantee that either did a good job, but you've got a better chance with a shop.