Sand is best
Use concrete sand under concrete pavers. Compressibility is more uniform during tamping. Also a better material interface for polysand. As far as washout, be sure to have your separation fabric run up to at least the sand height on your edges that approach step treads. Stone dust is good for real slate and flagstone as long as you use a depth that is flexible enough to accommodate variable thickness pieces of flagstone.i have pulled up many pavers laid on stone dust and replaced it with sand to homeowners satisfaction.