The Naniwa Stone holder is shallower to accommodate the thinner stones. For thicker stones the Steelex is fine, but for the Naniwa Selective stones the Naniwa holder will leave the stone exposed longer as you continue to use it. I normally get around 2500 razors honed depending on the Naniwa stone I am using. Sometimes more for the stones that get less work like the 12K which I only use for finishing and refreshing. Obviously, the Naniwa holder will work on the thicker stones too.

Have fun.