We have both.. except the 66 is a convertible and not a fastback.
we seem to drive the mustang more than the cobra. Mainly because of the 2 extra seats.
Now that said... If I'm just going out for a drive, to drive, the cobra is the goto car. I've been toying with selling the cobra and getting another 4 seat convertible but everything I've looked at lacks in the "smiles per mile" factor.
and I echo the 4 post lift. we put 3 cars and a couple motorcycles (and a tractor and snowblower) in a 2 car garage.
I'll also add that your mustang "should" appreciate as long as you own it. The cobra stays pretty stagnant in value (slight depreciation...then levels off)