Demographically speaking, I think the Viper WILL continue to appreciate exponentially while the Mustang, and especially a highly modified Mustang (regardless of how tastefully modified), will kind of track along with inflation-plus perhaps a bit. Just as the cars from the '20's, '30's and '40's kind of topped out (with some exceptions) when those who so badly wanted them as teens, grew up and entered, then left, their prime earning years, I see the same thing beginning to happen with the Mustang (and other "muscle cars," again, with the exception of highly original and low production examples). That and the fact that so many were built.
The Mustang has been, to a degree, immune from this demographic trend. But I've seen it lamented on here before that fewer young folks are taking up the hobby.
So, as an investment (plus for the joy of driving) the Viper (tho I really prefer the coupe) would be a good choice.
The caveat being that this is all JMHO, with nothing but anecdotal observations, versus real research, to back it up.
I'm with you on your line of reasoning...I see what you're saying and it does make some sense.
1965 Fastback 393W Stroker