QuietRock or Greenglue effectiveness in damping noise?

[Marian Smith]

How much difference does it make to add a layer of quietrock or use green glue to add a second layer of drywall to a 1960’s condo wall? How much better is it to open the wall and add rockwool insulation first? A small wall portion is open and there is something between units that looks like pressed wood fiber and it says “insulating”...I think it is some sort of sheathing. Any other ideas? Thanks