I would definitely go for the GT because no matter how many mods are made to the '11 V6 it still will not sound as good as a stock '05-'10 GT. If you decided to do engine, gear, chassis, and suspension modifications to a '05-'10 GT then it will definitely beat other GT's.
If you don't have the cash to spare to get the new 5.0 then get one of those '10 GT's because you will definitely be able to swap out parts from the '11 (if you're even wanting to modify the engine).
If you are looking for better fuel economy then get that '11 V6 because no mustang has ever been able to get such great fuel economy and have that much horsepower.
'66 Coupe 200 cu in | Candy Apple Red | Black Interior | Rally lights | A/C
'12 VW GTI 4 dr | DSG | Neuspeed P-flo intake | pushing 225 whp
SOLD '11 GT Prem Kona Blue | Saddle | Sec Pkg | Brembos | had many mods