You can just use standard wall insulation. If you want a little extra, you can use R19 in a 3.5" cavity but not really necessary. Roxul will give a small improvement in isolation but not a lot for the money spent.
It all depends on what the other side of the wall is exposed to. If it's going to be drywalled, then either do that, or better, build 2 separate walls and drywall only the outside of each wall. This provides better isolation and also deepens the insulated cavity which drops the resonant frequency of the structure.