I like them both pretty equally.
It's a shame that a non-running, rusting shit bucket goes for $15k, so I'll probably never have either one.
Truth be told that $15,000 rust bucket isn't even the better value anyway.
My preferred way to go would be to find that 318 car in great shape. You'll pay more for it but the tough part is done. You can then find what-ever you need for power later. Most of us are better at that part anyway.
A Classic Road Runner or Satellite doesn't have to be crazy priced. The road to happiness and a reasonable price comes from tossing out the play book that comes with "matching numbers" and the popular collector engines.
I took a quick look a prices and while a 440 or Hemi car will run North of $50,000 and many in the neighborhood of $70,000 in good shape.......there are some pretty good looking 318s out there for as little as $20,000. At these prices there is no way I'd take on a $15,000 rust bucket either.
I'd spend a bit more to get that solid body. I'd treat it like the "blank slate" for what-ever I want under the hood. Crate motors available today are light years ahead of the old engine tech. You don't need computers or fuel injection to see big gains. New head design and roller cams just work so much better than what was available 50 years ago.
You really can't do that with a car that has already achieved a crap load of Collector value as a matching numbers 426 or 440 car.
I'm thinking that if you shopped this hard and stuck to a plan. If you do all the wrenching, you could put your dream car Satellite together for the price of a New Challenger and you'd have a great looking car that could easily kick ass vs an original 426 or 440 car that really couldn't be driven hard anyway for fear of "breaking" something BIG and decimating it's purchase price value.
Best of all when you want to sell it on down the road you probably get a good chunk of the $ you spent back. Try saying that about a new Challenger 10 years or so on down the road.