Having owned most of the late model Mustang variants (and having driven at least one example of all of them), I have to admit that I would be torn. The S197 is definitely a superior car, but I do have a soft spot for the comparative rawness of a Fox or early SN95 car. The S197 is almost too refined.
$20k to spend on a car? I'd probably buy a S197. But I'm burnt out on projects. However, at that kind of money I'd be tempted by a Fox along the lines of Craig's coupe (prior to the current disassembly).
Faster, faster until the thrill of speed overcomes the fear of death.