live rock rubble in the sump is good because it gives a great little area for critters to breed and be safe from fishy predators. Also LR rubble has smaller mass than big bits so a higher percentage of it is surface area and pourous so good for the bacteria.
the sand in sump i assume you mean deep sand bed? this is where you can use non anerobic bacteria as part of the filteration. its kept in the sump as if its disturbed it could potentailly wipe the tank