If you like modding, the Mustang is 100% the car for you. Parts come relatively cheap and extremely easy to install.
Both the V6 and V8 are impressive in their own way. For $30k-$35k you will arrive at about the same HP with performance upgrades, whether it be the V6 or V8. The biggest difference is that with the $30k ish money you would have either a voided warranty on the V6 or a bone stock v8 full warranty (until it's modded).
With all the troubles some people are having with the transmission, giving up the 60k mile warranty for a few performance upgrades may not be the brightest idea. But then again, the transmission problem only seems to be popping up in a small %, so it would be 'at your own risk'.
However, if you are buying used. The GT holds its value better than the V6, unless you get a killer good price on a used V6.
One more important note: A lot of people are having problems with damage to cylinder 8 (you guessed it, in the GT) with a tune. There have been no complaints on the board about tuning the V6.
I do autocross and I would much rather run the V6 PP than the GT. The grip is better in the V6 PP than any of the stock GT models (weird, huh?), and I am running in G-Stock rather than F-Stock, with a better handling car.