[Subsea]

With respect to substrate in your refugium, I would use both. Miracle Mud provides a fine silt to be home to worms, but it is not alive. Some MM is a source of iron. In my mud/macro refugium, I used FORM (funky old reef mud) and an iron rich substrate from Seachem. Live sand brings in diversity of bacteria and detrivores.

Depends on the quality of the live sand, but if you are limited to only one of the two, live sand is the winner.