I've gone thru a variety of cleaners.... all of the following is from my own experience.
Hermits - PROS excellent scavengers of leftover food (and dead livestock sadly) and some will eat certain types of algae, I have personally witnessed my dwarf blue legs eat hair algae, very low risk to fish. CONS - will kill snails and other crabs for their shells even if you have tons of empty shells for them.
Emerald Crabs - PROS many will eat hair and bubble algae, very low risk to fish. CONS have been seen picking on some corals and clams
Trochus Snails- PROS very low risk to livestock, can right themselves, consumes lots of film algae, cyano, and diatoms. CONS doesn't do much for nuisance algae.
Astrea Snails- PROS loves nuisance algae, low risk to livestock. CONS cant right itself if fallen over, not a good climber
Margerita Snail - PROS seems to eat just about any algae, very low risk to livestock. CONS short lives at most reef aquarium temps
Cerith Snail- PROS smaller snail able to get into small spaces, eats just about anything, comes in a few different sizes (dwarf or florida). CONS apparently delicious to hermits, small pointy shells are perfect size for clogging intakes/surface skimmers.
Nassarius Snail - PROS hide under the sandbed for safety during day, cleans detritus, uneaten food/fish waste. CONS doesn't eat any algae, always hides so its hard to tell how many you have.
Stometella Snail- PROS reef safe, eats microalgae/film algae. CONS only active at night, easy prey for wrasse, not usually sold in LFS
I know you were really asking about snails/crabs but you may also want to look at adding shrimp and maybe some sort of sea cuc or sand bed cleaning critters.... anything to reduce nutrients will help with algae control.