I personally have not had crayfish (yet) but they are both opportunistic bottom feeders and have been known to eat just about whatever they can get their hands/claws on. In general I don't believe the care requirements are too different, but specific crabs and crayfish will likely have some variances.
Yeah I just posted some info. regarding crayfish on another post: Maybe check it out! Ehh crabs have roughly the same requirements as crayfish, although they tend to be a bit more sensitive when it comes to changes in water etc.