Vipers are OK if you enjoy breadtruck exhaust sounds, wide door lips, clunky non-crisp handling at low to normal speed, styling very similar to Dodge Stratus (03 up) etc. May not be, but feels big & heavy.
Ford GT...Only made for 2 yrs. Nothing else looks like it (one of the best styles ever...timeless imo) No big ass heavy V-10, just a really fast, awsum sounding V-8 as American muscle should be. Much easier to work on. Handles incredibly, very nimble Will draw ten times the attention (if you are into that type of thing).
I have owned both and never had anyone hang out a window to take a picture of my Viper......I think that says something.
My money on value in 20years is going Ford GT all the way. And that my not be a factor to you either.
Like a wife of gf ...you have to look at it often. Ford GT is gorgeous, sexy, easy on the eye....a work of art. Viper.....uh, not so much.