Hmmmmm...
The exceedingly low mileage on the two Mustangs is really the only thing that is holding me up from offering an opinion...
If the Cobra had, say, 40,000 miles on it, it would be low mileage, but I would readily say just put gas in it and drive the hell out of it. But with 1,500 miles, you are kind of in the "you don't see too many of these cars in this condition anymore" territory...
That being said, I would still drive the hell out of it, only because you have the even more desirable and even more insanely low mileage Saleen...
That one I would prolly park and keep pristine. Drive it to shows, take it out on a Saturday afternoon cruise. I wouldn't start driving it a thousand miles a month though. I wouldn't drag race it every weekend. Not out of any hope that it will be worth some insane price in 10 years, but just because some things are nice to look at...
All things being equal, I personally do not think cars are that great of an investment if you are looking to make money LONG TERM. The chance that they will appreciate faster than the rate of inflation is pretty low until you start getting MANY years out. And even then, you need the exact right set of conditions, namely limited quantity, uniqueness, and desirability. I think desirability is the main problem with more modern cars as far as collect-ability in the future will go. Not that people do not want these cars; just that they do not carry the same level of nostalgia as, say, a '64 does to a guy who remembers his dad taking him for ice cream with the top down on a Sunday afternoon...
But, you definitely got a killer deal on those vehicles. You could more than likely turn them around relatively quickly and make some money right now. If making money is your primary concern, then I say put them up for sale right now, because it seems unlikely that waiting will increase your profits.
So, that is my "stream of consciousness" about the cars. I would personally keep the Cobra to drive, keep the Saleen just to have, and get rid of the others. But, that is just me, and what I prefer and like is obviously different than what you do, so, really, only you can decide what is the best course of action for you...