Look, this topic has, and probably will be, beaten to death for as long as there is the automobile. Mufflers are not known to give you large HP gains. The biggest benefit is less backpressure which, when coupled with other mods, can help give you a LITTLE extra HP.
The biggest reason I am going with a Flowmaster setup is one reason and one reason only. Ever since I was 16 years old, 16 years ago for those mathamaticians out there, I have always wanted a Mustang. I wanted one for their looks, their potential to stomp on most anything out there with a little tweeking, but most importantly, the trademark sound that only a Mustang can make. Now that I finally have one ('04 GT) I can't think of any other sound I want coming from the rear of my Pony. Who cares if they are not giving me 1-2 hp more than a Magnaflow system would. I just want to have that sound to drool over each and every waking day for the rest of my pathetic anal retentive life.