Hmmm.... It's a tricky one...
I will try and be unbiased.
For me, although they both appeal to me and if both cars were in front of me, I would go for a 156 GTA over a GT!
Well although they both drive similar, similar in value too.... It would be the potential classic nature and eventual value and status that the GTA will have in years to come....Just look at some older Alfa's now and the values that they can achieve, yet some can't as of yet, cut the mustard.
You look at a GT and the only real tell tale is the badges that make the 3.2 special.
On the other hand, you look at the GTA and you just know it is something a little special and that will eventually give it cult status in years to come.
You just have to look at other car makes for this phenomenon:
Ford Escort RS1800 mk11 or a droop nosed RS2000... The latter being almost twice the value of its brother.
Lancia Beta coupe or a Delta Integrale EVO... Same again
People love the older top of the line cars that were performance orientated and stood out with that certain "je ne sais quoi"
Has the GT actually got that?.... It is certainly beautiful in it's own right, but has the 3.2 got something that visually stands out from its brother's and sisters?