There is a video on youtube that has like 5 different exhausts on it (in the same video, so you dont have quality change form camera to camera). I used that video and a few opinions to help decide what I wanted. I will have to post the youtube link later, as I am at work now and youtube is blocked.
I did not go with flowmasters, IMHO they are the most mellow (worst sounding to me) aftermarket exhaust you can get for a mustang. I went with the SLP loudmouths. Idle sounds good, cruising there is no droning, and acceleration is wickedly loud. Its close to the sound I was looking for... I need to get rid of the x-pipe and replace with independant pipes (true dual) and go cat-less to get that, but thats besides the point. I am very happy with the loudmouths.
Sounds to me if you dont keep the stock setup, Corsa might be what you are looking for. I found Bassani, JBA, and Borla also to be pretty mellow and quiet.
Here is the link to the thread I posted
http://www.allfordmustangs.com/forum...h-exhaust.html
Hope this helps, just my 2 cents.