HAP40 and ZDP189 are not really comparable. The only thing they have in common is that they are from Hitachi and that they can reach similar hardness. The chemical composition is:
ZDP189 C 3% Cr 20% Mo 1.3% W 1.5%
HAP40 C 1.3% Cr 4% Mo 5% V 3% Co 8% W 6%
HAP40 is a highly alloyed tool steel, not stainless. ZDP189 was developed as knife steel. Of course you can make a knife from every steel, but given proper heat treatment, ZDP189 should definitely perform better: smaller carbides, easier to sharpen, superior rust resistance.
Have no direct experience with HAP40, but once tried a Kiya knife made of HAP72, hardened to HRC 69. Didn't like it at all. A pain to sharpen, prone to chipping and reactive like Carbon. No match for a Cermax.