too many exhausts to choose from!

[Rollinastang]

I got an 03 convertible v6 automatic. just put a K&N intake on it last summer. when i have the car in park or neutral and rev the motor there is a slight sound when i let off the gas as if it's not releasing the exhaust fast enough, also when i shut her down there is that same sound.
First question is will a cat back dual exhaust fix that stupid sound or should i take it too the shop, this noise didnt happen until i put on the air intake so i am assuming i need a better exhaust. and what brand is the best i want a deep sound all the way around not just at heavy acceleration.When the car is idiling i want it to be heard you know what i mean?
Alot of people i talked to online say flowmaters are good enough but i am kinda stuck on Borla. There is a quite large price differance between the 2 and i dont wanna spend more money if they both are gonna sound the same in the long run. Also what about MAC or Bassani i havent heard too much about them but i seen them? Reminded i have to keep it emission legal so off road pipes are no option.
Thanks for your help!