Which of these ponies is easier on the wallet in both the maintenance and long term costs? How much is it for one in fairly good condition but still needs cosmetic/interior work? And finally, how common are restoration parts for the cars?
You can build a Mustang practically from scratch these days, everything is available for the early models.
The early Barracudas aren't very popular, but since they're basically fastback Valiants they would be cheap to buy and own.
There seems to be more reproduction parts for the Challenger all the time, but the E body Mopars (70-74 Cuda and challenger) have become so popular lately that even really sorry examples can be expensive to buy.
Personally I would choose the Challenger, because I miss my 72 more than I miss my 65 Mustang.