From what I have seen, a lot of pet stores keep the chips in their cages, but I personally like the sand and gravel. It is much easier to clean and also cheaper to replace when the time comes.
Crabs also seem to find sand and gravel a lot more comfortable.
Wood chips are harder to bury themselves in like they will do when they decide to molt. Sand in at least half is the best advice I can give. From what I know, the wood chips may provide humidity, but only by keeping them moist, and if you keep wood chips moist, they also will produce mildew. Not something you want your crabs smelling like. Hope this helps.