A lot of the answer to your question depends on how you use XTOL. Full strength, XTOL is a solvent developer, much like straight D-76. They both dissolve some of the silver on your negative in an effort to make it look finer grained. The finer grain effect comes at the expense of sharpness. As you dillute either developer, you get less of a fine grain effect and more sharpness.(posted 8380 days ago)PMK takes a different route. It delivers all the grain that was built into your film in an attempt to achieve maximum sharpness. The yellow-green stain PMK leaves behind helps in two ways: (1.) it fills in some of the gaps between the grain, making the film look less grainy, and (2.) it acts as something of a intensity-based highlight filter with variable contrast paper, helping contrasty highlights to print more easily.
One caveat. Papers with an extended range and lower highlight contrast (like Ilford Multigrade IV) can look flat when printing a PMK negative. With most papers, however, I find PMK really "sings," especially in pictures strong in highlights like whitewater or mountain snow.