"Not having tried Pyrocat and being pretty experienced now with PMK, what does pyrocat do differently than PMK that I would want to try it?"
PMK is an excellent developer. I used it myself for almost ten years as my primary develolper so I am very aware of its characteristics. Pyrocat-HD gives very similar results to PMK: high acutance, grain masking from staining, and high proportional stain. And the stock solutions also have very good keeping qualities, with a slight edge to PMK in this category. However, Pyrocat-HD has some characteristics, which may offer advantages.
1. Pyrocat-HD can be used with rotary processing with no fear of uneven staining. I recommend it for rotary processing and have never had any reports that it caused uneven development or staining in this application.
2. Pyrocat-HD can also be used with minimal agitation when processing roll film in tanks. This can give very pronounced adjacency effects and result in enhanced apparent sharpness. Pyrogallo based developers, on the other hand, are subject to uneven staining when processes with minimal agitation.
3. Pyrocat-HD, even at the 1:1:100 dilution, requires shorter development times by about 25% compared to PMK to reach the same CI.
4. Pyrocat-HD is less susceptible to fog from aerial oxidation and even with very long development times keeps a very low b+F. This make it very attractive for alternative printing processes that requires negatives of very high CI.
5. Pyrocat-HD gives slightly greater effective film speed than PMK, when negatives are developed to the same CI. This speed gain is due to Phenidone.
6. Pyrocat-HD is a better developer for making dual-purpose negatives, as for printing with both graded silver paper and Pt/Pd with the same negatives. This is because there is greater difference between the blue reading (to which silver papers are sensitive) and the UV reading (to which UV processes are sensitive) in a stained Pyrocat_HD negative than in a PMK negative.
7. There is a difference in the way highlight values are rendered with Pyrocat-HD because it does not have the shoulder in the curve that one sees with PMK. This is neither a good thing nor a bad thing because the shoulder helps with certain scenes by providing compensation, but with other scenes it gives reduced highlight contrast. High mid-tone values are also rendered with slightly more contrast in Pyrocat-Hd for the same reason.