I am leery of snails. I think their bioload is understated. Ever see a garden snail slime? And, it takes a lot to make a dent. Then, you have the potential of a die off after your tank is clean.
Love the mithrax crab, though. Stuffing his face constantly with what is barely perceptible to humans. Goes where snails don't venture. Also, This guy cleans places I can't reach with a toothbrush.