I personally feel that the New Edge cars will increase in value substantially by the time they are old enough to fix up as classics. They will forever be remembered as the last, and arguably best looking bodystyle Mustang of this era before the current muscle-car market went "retro" crazy.
As for Foxes, they are popular and have a big aftermarket because they are (flamesuit ready) a dime a dozen. You can pull any s***box Fox car out of a junk yard, throw a lil money at it and have a fast car for next to nothing. You don't see as many SN95 platforms because they are newer and havent had as much time to get ragged out and forgotten so that one day Joe Mechanic can drag it to the 1320.
Granted, there are a few exceptions, like the lighter weight of the fox, and the "cheaptogofast" nature that only pushrods could provide. And in general while they are older, they are much like Jeep's XJ Cherokees in a sense. Sure they are a dime a dozen, and sure most of them have taken a beating, but in the end it is a proven platform that gets the job done, without breaking the bank (modular cars are quite the opposite, at least for the time being).
As for the cars that will hold value, here's my take, and I'm going to have to agree with Kilgore. As far as the SN platform goes, I don't see your run of the mill GT's going for anymore that your typical hatch LX Fox goes for nowadays.
Cobra, Bullitt, Saleen, Anniversary, Etc. cars will always be worth a little more, and of course the super low mileage all original cars will stand out as well.
I think that by the time my generation (I'm currently 21) finds themselves midway through their 40's, the SN95 platform modular cars will prove themselves to be a worthwhile car to have in your garage.
*side note: i just hit 200 posts, and boy I just wrote a small novel*