First of all, there are many pyro developers, so more specificity is needed, and "best" is probably not a useful word here, as it is too general, and can be quite subjective.
In my short experience with PMK pyro, it stains more extremely when using films with a traditional grain structure, i.e. not T-Max, Delta, or Acros. So you might say it "does it's thing" more with films like HP5, Tri-X, FP4, Plus-X, etc. This is not to say that it doesn't work with tabular-grained films. It just seems to stain less, thus it masks less.
X-Tol is well known by many and it is available pretty much everywhere. Pyro is not as universally known, and not as commonly available off the shelf.
X-Tol is a pour-back, one-shot, or replenished developer. Every pyro developer I know of is one shot only.
Neither will give you unacceptable grain or sharpness for most things.
Pyro should be used with an alkaline fixer to obtain the most extreme stain. X-Tol can be used with any fixer without any possible drawbacks.
Pyro is a highly concentrated developer, used similarly to HC-110 or Rodinal, while X-Tol is used more like D-76.
Pyro can be useful for alt process printers who also print on silver paper, as the stain works as a spectral highlight mask on the higher tones when printing on VC paper, while it works to boost the contrast on UV-sensitive processes like cyanotype, VDB, and platinum, which need higher contrast negatives than your typical silver print. It is is kind of like having two negatives in one...all through the magic of the spectral sensitivity of the printing material. Of course, different types of pyro produce different colors of stain, so they will do this masking to different degrees with different printing materials
Pyro takes some more care in the lab, as it's ingredients are more toxic. X-Tol is a very safe developer in the grand scheme of things.
Both create good results with pretty much any film, though different. It would be nice if the Photographer's Formulary offered a small 4 or 5 batch sample package of PMK and Pyrocat HD so you could see if you like it or not. It is not for everyone. For general purposes, it is not for me. It requires too much special handling, and I am accustomed to the "snap" in the high tones that I get with more standard developers. I use pyro in certain circumstances only. But some people use it for everything, so that certainly is an option.