First, pyro by itself usually refers to pyrogallol. Pyrocat HD uses pyrocatechin, AKA catechol or pyrocatechol. The stains are of different color. Both stain images are more dense to UV light than to visible. One of the advantages quoted for pyrogallol, specifically PMK, is that it can be used equally well on VC and alternative process. The alternative processes generally require a higher contrast negative because they are printing-out contact processes that are self masking. The yellow stain image adds much more to the contrast under blue or UV light than it does to visible or blue-green light. The differential may be in the other direction when comparing pyro prints on graded paper with alternative processes.

Pyrocat may not be as much different. You will have to ask Sandy King.