Flowmaster's Sound really nice below 1800 RPM but are almost silent at highway speeds. I don't notice any additional noise over the stock mufflers. I have the 40 series, dual chamber mufflers.
I've got a MagnaFlow Axle-Back kit on my '06 GT. It sounds Great! (if you don't want a REALLY loud sound.) It has a deeper sound than stock, but it's not as loud as the Borla's. On the inside of the car, it "sounds-off" when I accelerate and "quiets-down" as I begin to cruise; there isn't much of a drone either. Everyone tells me that it sounds Great, but don't get the Magnaflows if you want a LOUD sound. check out the soundclip @ magnaflow's website: http://www.magnaflow.com/04sound/04american.asp
Clip Name: 2005 Ford Mustang GT - High Quality Audio Files