it all depends on what you are using a fuge for. check out the book Reef invertebrates. it really has the more information on fuges than you ever need to know.
for a basic run down, if you want to grow algae, a simple sand bed will be fine. for denitrifacation, a deep sand bed is needed, the deeper the better. if you want to grow infuana and copepods and other plankters, a rock rubble base is the best. you only really need the mud if you want to grow actual marine plants like mangroves, but even that is debatable. HTH