Get a good mix. Different snails do different jobs.
Nassarius and conchs - good for stirring sand bed.
Turbos - good for rock.
Astreas - good for glass.
Cerith - good all around.
Hermits - good for detritus.
My favorite are zebra hermits and scarlet reef hermits.
Cucumbers - good for sand bed, but need a large sand bed to keep alive.
Sally lightfoots - good for long algaes.
Emeralds - good for something I'm sure, but I don't like them so I don't keep them.
Urchins - good for algae on rock and glass - but they do not discriminate - they eat all algaes - including coralline.