Margaret, the only difference I am aware of is that the 590 Night Stick model has a wood grip and forend whereas the normal Shockwave has black polymer furniture.

stock and forearm 1 is wood (night stick) and one is polymer

Margaret, the sole difference is the wood furniture versus the synthetic stock.

Primarily, the wood grip and fore end versus the synthetic grip and fore end. As both are Mossberg 590s, they are mechanically the same. And of course .. the price difference. :-)