I've had the stock muffler, a single GT muffler and now i've got a Flowmaster American Thunder single. Here's what i've noticed along the way.
The stocker was boring and seemed to choke the motor.
The GT opened the motor up a good bit. I heard an increase in sound with no drone at highway speeds and nothing too crazy sound wise. It was a bit louder but didn't rattle the teeth out of your head nor did it sound exceptionally deep. I felt a fair 'kick-in-the-ass' performance increase when I installed it.
I soon got bored of the GT takeoff and bought a Flowmaster American Thunder single. After installation I noticed that the exhaust was just a bit quieter than without a muffler and it sounded a lot deeper and with more balls. The Flowmaster also has a black body so it blends in with the underside of the car and it sports a 3" rolled tip which fits perfectly in the cutout of the bumper. I noticed another power increase when I moved up from the GT takeoff to the Flowmaster.
Its all up to you performance wise. Pretty much any of the big names will give you the same performance. Borla, Flowmaster, Magnaflow, Pypes, Mac will all give you pretty much the same performance for varying prices. I believe the Flowmaster is the cheapest of all of them.
If I were you i'd skip the bull and go for a good muffler. You're going to end up getting bored with the sound and the performance and start looking for something else eventually.

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Black 2005 Mustang v6, 5 speed, IUP, EUP and Leather.
Razzi 1/4 window scoops, Mustang GT Sway Bars, Flowmaster American Thunder Single Exhaust, Clear Front Turn Signal Lenses, Demolet CAI, X-cal2 91 tune, Hurst Comp/Plus Shifter, FRPP T-Lock, Ford 3.73 gears