You don't need either. I've seen no evidence of mud, miracle mud, whatever being of any benefit and seem to recall someone saying that when tested it is pretty much stuff straight out of a garden.
If you have a suitable sandbed in your display there won't be any additional benefit from having more sand in the refugium.
Having a bare bottomed refugium will help with keeping it clean if necessary.
If you want to keep macro algae that requires being planted in sand then you could plant in a small plastic container to keep it clean.