cat-back system refers to many different things. A Cat-back system can be a single exhaust system or a dual exhaust system (True Duals). True duals, too me, are much better looking and better sounding than a single exhaust or an exhaust with a connector pipe (H,Y,or X pipe).
For your 2001 v6, I'd have to recommend the same setup I've got. If you got any ma and pa shops around your area that can fabricate the pipes for your
stang I'd absolutely recommend it.
What I did was buy mufflers off of ebay and just had the exhaust place fabricate the piping to fit my mufflers. I'm running true duals from the headers back and absolutely love the sound.
I bought Pypes Violator Mufflers off of ebay for $40 total. I can't find any more deals like that on those mufflers but I do recommend Brenspeed's Ebay store for the mufflers. I think they're $130 shipped for the two mufflers brand new.
Here's the mufflers
Just find the cheapest ma and pa exhaust shop around that will fabricate the pipes for you instead of order them, getting them fabricated may take a little longer but its MUCH cheaper than having the exhaust shop buy them for you.
If you want to hear what my mufflers sound like go here:
Garages > Schumi23's 1998 Ford Mustang - FQuick.com
Or if you want to hear it on a New Edge Mustang (99-04) clicky here:
Basically exhaust all depends on the person that's driving it, so it's up to you to decide, check out youtube, google video, and other sites for exhaust sounds.