GT of course! I don't think you'll get too many saying go for a 3.0. Love my 07 GT, but my 13 3.6R is just as nice in a different way. Both have good power, but the turbo on the GT is nice when it kicks in
If you don't plan on modding, I am sure you would be happy with the 3.0. GT will be a bit harder to find with lowish miles and a decent price, but they are out there. I had no plans on modding mine, but since I have the 3.6 also that has gone out the window. 75k, about time for more power.
Evey time I pull up to the dealer with my GT for an oil filter or something, the sales guys always look. Even the sales guy that sold me the 3.6 kept asking if I was trading that in. There he mentioned there was a copper colored (I believe) Spec B coming in, but one of the sales guys already has dibs on it.
If you go with the GT see about service records, when the turbo was replaced (if) and make sure you are on top of your oil changes. Also make sure to get it inspected, compression test, etc...
Drive both and decide.
I say go with the GT.