The Dart will likely be worth more once fixed, but it may end up costing more to fix and not just based on the quarter or the rust. Some things to consider...
If the '74 Duster was originally a 318 car, it will also be a BBP, disk brake car (at least originally). The BBP brake and large ball joint suspension parts for the most part are cheaper to rebuild/replace than the SBP brake and small ball joint suspension you will find on the Dart.
The '73+ model interior pieces, primarily all the plastic- door tops, sail panels, fold down seat panels, etc - are not reproduced and are usually cracked. The sail panels especially are difficult, because of their large size they don't ship well even if you can find a set. So if the interior of the Duster is good, that's good. If it's all busted up, it could be difficult to find the plastic pieces in good shape.
Not sure what engine the Dart had, but it might have an 8 3/4. The Duster would be a 7.25 or possibly an 8.25 rear, depends on how it was set up. If it's an auto car it's most likely a 7.25", but it will be BBP because of the front disks that would have come with the V8.
You probably already know this, but if the Dart has a ton of bondo in one place, it's likely to have it other places too. Sounds like both were repainted though so who knows.
The '73+ cars do seem to be gaining value, I think less people care about the redesign because the value of A bodies has improved in general, so folks are getting priced out of the more popular models. And yeah, mechanically the later cars are a better starting point. Not sure what your plans are, but the BBP brakes make a lot things easier when it comes to reproduction parts, wheel options, brake options, etc.