Some snails may eat plants present in your aquarium...If you choose snails, make sure you dont just leave it to eat scraps and algae off your tank, you should supply it with vegetables as well as a algae wafers every now and again...This may prevent snails from starvation as well as being tempted to eeat plants in your tank...
Ottos on the other hand have been known to eat algae readily off your tank...Ottos are sensitive fish and you have to make sure you tank is fully mature before placing any ottos in your tank....Similar to all bottom feeders, algae present in your tank should not be its main diet and should be supplemented with various vegetables and algae wafers tp prevent starvation....Hope this helps.
P.S. just to let you know some snails such as apple snails are heard to grow to a size of a tennis ball when fed correctly.