Are you planning on gaining any performance out of this or are you wanting it solely for sound? One thing you must consider is the mid-pipe. From the factory, 'Stangs come with H-pipes. H-pipes are good for that deep "muscular" sound (along with a slight improvement of low-end power over an X-pipe). Sure, with some resonators, you will get
some of that race-car sound you are looking for, but you won't acheive its full potential unless you get yourself an X-pipe. X-pipes are good for more of a race-car sound (along with a slight improvement of top-end power over an H-pipe).
What are your intentions with the car? A daily driver with some noise to it? Or a car to kick some serious ass?
An H-pipe is what you need if you are going to be doing some slight modding and just want that signature '
Stang sound.
An X-pipe is what you need if you are planning some serious mods in the future since it will result in the most power out of the two. And an X-pipe will give you that race-car sound too.
Now that I've gotten that out of the way. Muffler vs. Resonator:
I am guessing when you say "muffler" you are referring to something with chambers to it? And a resonator being something with a straight-through design where you can look through one end and see whatever it is on the other side without and obstruction? In that case, a resonator will give you a "race-car" sound. Race cars do try and get the most free-flowing exhaust right?
Hope that sheds some type of light on the subject...
-Tyler