I would definitely consider a 13-14 Shelby - they're light years beyond what comes to mind when you think of typical old Mustangs. I have the Shaker system and it sounds decent, I guess, but still not as good as maxing out a credit card at Best Buy or whatever place is cool nowadays.
Microsoft SYNC is amazing, to me.. but I come from a long line of caveman 90's cars. Being able to sync up my phone to the radio for Pandora or make phone calls from the steering wheel blows my fucking mind. You also get a Tremec 6-speed, so transmission failures are not a worry like some believe the GT's MT82 to be. Also, 662 fucking horsepower should be enough for a while.. if not, the sky's the limit in modding. The best part is these cars are amazing for plain old daily grinds to work as well, as the traction and stability controls are some of the best I've ever experienced with rwd. For $55k (new) that's a hell of a deal, imo.
Cons:
Stock handling is poopy.
Back seats are great for backpacks or toting a six-pack home from the gas station, but that's about it.
If you're married you can't ethically fuck all the bitches that want your Mustang dick.