A decoupled wall will always out-perform a single wall system for sound-proofing purposes.
It's best to determine precisely what you need because more isolation = more cost. There is no 'one-size-fits-all' when it comes to studio isolation.
Kindly go to my resources page and download "How To Find How Much Isolation You Need". I think that will answer all of your questions. To find out how well certain wall systems perform, you can also download the 'ir761' which shows all the testing data for many types of frame walls.