Ford is doing amazing things with turbo engines these days. I'd test the turbo 4. But I've had several v8 mustangs and the sound is always great. Still.... Turbos also sound good and have excellent power + fuel economy. Lower weight etc
That is my only issue with the Ecoboost 2015 Mustang. It isn't much lighter than the Coyote equipped GT.
Mustang EcoBoost: 3,532 lbs (manual), 3,524 lbs (automatic)
Mustang V-6: 3,526 lbs (manual), 3,530 lbs (automatic)
Mustang GT: 3,705 lbs (manual), 3,729 lbs (automatic)
173lbs is not a huge weight advantage when opting for the Ecoboost. Also if you look at standard features the GT has bigger brakes and wheels. I am guessing there is at least 30-50lbs of added weight in the GT that has nothing to do with the motor. So the actual weight of the cars in optioned form is closer than 173lbs.
Front Brakes = EB 12.5" (optional 14") vs 15"
Rear Brakes = EB 12.5" (optional 13") vs 13"
Wheels = EB 17" vs 18"
You can't go wrong with either motor, but you certainly are not gaining a significant weight advantage with the turbo-4.