[If you are choosing one or the other, no doubt go with snails. You can do both but many times your crabs will begin slowly munching on your snails which is probably why you have read to go with one or the other. Many people successfully have both. I have both and have had both for over a year in my big tank.
Having said that I will never add another crab to any tank again. I simply think they are not neccessary and sometimes destructive. A good mix of snails can do anything you want!]
