MAC cat back or Flowmaster catback....
 

which sounds more MUSCULAR when combined with a high flow catted H-pipe?

I,ve had both. The MAC flow path stright through design sounded more "turboish" is the closest I can explain, or maybe I should say closer duration exhaust tones while the Flowmaster has the classic "older Muscle car" drone or more spread out exhaust tone. The flowmaster seems to be loudest between the 1800 to 2200 RPM range. The Flowpath sounded real good but I personaly like the old car sound since those are the days I was brought up in! OF course both systems were loud and sounded real mean when you get on it!!
:D

Get the Flows. Like S/B said, you can't beat that old school Flowmaster sound. The MAC's are much quieter than the flows by around 12 db. Just don't get 3-Chambers (Force II system) get 2-chambers (Amer. Thunder system) because the 3-chmabers are barely louder than a stock GT if any.

I've got the MAC flowpath mufflers with the prochamber. It gets more comments from people on how awesome it sounds. I put the mufflers on before the prochamber, and it still sounded pretty good with the stock h-pipe. Flows sound good, but MAC sounds awesome also. I didn't want to sound like every other mustang that drove by. Not that they sound bad, but I wanted to be unique.

I have a 3 chamber flowmaster cat back system with a high flow X pipe. It is reall loud and sound excelent. As far as getting the 2 chamber Im not sure...The 3 chamber is about as loud as I want the car to be. But I supose its all a matter of taste. I would go for the Flows. Well good luck.

Bob

I have a Mac off-road X and Mac cat-back. It screams when you get on it and isn't obnoxious when your cruising, unlike Flowmasters. I went with Mac just because I got tired of hearing Flow's on every Mustang out there. It's a nice change.

I have flowmaster 40's (2 chamber) my buddy has the mac mines a little louder but his has a more "consistant " sound unlike the flows that have the garrgle that sounds so sweet

Re: MAC cat back or Flowmaster catback....
 

I Would Go With Flowmaster, 40 Series, American Thunder. I Currently Have Them On My 86 Hatchback Without Tailpipes And Believe Me These Are Aggresive Sounding. I Make Alarms Beep As I'm Driving Past Parked Cars. Don't Get Me Wrong, It's Not A Nasty Sound, It's A Deep Powerful Tone That's Gotten Me Several Compliments, From An Almost Stock Car. I'll Be Installing The Tailpipes Here Pretty Soon, As You Tend To Lose Top End When You Run Without Them. But I'd Definitely Go With Flows. Flows And Stangs Are The Perfect Combo.