basically, harder wheels have less grip and softer wheels have more grip. some people like less grip so they can slide and some people like more grip so they can carve.

hard wheels will go faster on smooth ground but will be nearly impossible to ride on rough roads. they will not be able to conform to the irregular surface and the whole board will vibrate so hard your brains would rattle out your ears. you will need big soft cruiser wheels to do any decent speed on rough roads.