As far as discounts go, decide on the sound you're looking for and then search out deals.
There is an AmericanMuscle club discount, I believe is 5%. Sometimes items aren't eligible, but I believe most of the exhausts are. You enter the code mentioned in this thread:
http://www.newyorkmustangs.com/forums/click-official-ny-mustangs-american-muscle-discount-%2Afixed-5-31-13%2A-t2639.html
Then when you go to checkout, enter the code, and the discount will be applied, if applicable.
For me, I wanted a deeper note as alot of exhausts for the 5.0 tend to get raspy.
I went with the RTR mufflers, I think they're now called the Street spec version. When I got mine, they only had one version. Now there's also the Competition Spec which is supposed to be louder. It's made by Magnaflow, so it's of decent quality, but it wasn't cheap. $~650. You can get the tips in black or chrome. The sound is adequately loud in person up till about 3500 rpm and then it kind of tapers off and gets quiet.
Here's what it sounds like on a stock 5.0:
https://www.youtube.com/watch?v=mo6qJ6GN8Yk
Here's what it sounds like on a 5.0 with an X-Pipe:
https://www.youtube.com/watch?v=KKZEgWuxQMw
As far as drone, it's tolerable...but thats to personal preference. I only experience it at about 2,000 rpm, under load, going up a hill ~70mph. At 65 and 75, it's almost gone.
From what I understand, the louder end of the spectrum would be the Roush Axle Backs.
Then finally, if you're set on a Cat-Back system, here's what AmericanMuscle has to offer if you choose to go through them.
2010-2014 Mustang Cat-Back Exhaust | AmericanMuscle | Free Shipping A good number of them have audio clips to give you an idea of what they sound like.
Personally, (if possible) would go out and see some real world examples, i.e. car shows/friends because while people try to capture the sound on their camera's, the sound will only be as good as the microphone used.