I have tested and used both developers extensively. I have detailed tests on Perceptol at various dilutions with TMX 100, Delta 100, Acros and PanF+. Perceptol is my primary developer.
These are not the same type of developer so comparing them in terms of "better/worse" doesn't make sense. XTOL is a Phenidone/Ascorbic acid, well buffered solvent developer. It gives full film speed and actually up to about a half stop more, so it can be classified as a speed enhancing developer. At stock strength, it is both slightly sharper and finer grained than D76, with similar tonality. As you dilute it, behaviour is typical of dilute solvent developers - ie increased acutance and increased grain both due to reduced solvent effects. Indeed at 1+3 XTOL is quite sharp, and the grain increase is comparitively small due mostly to XTOL being well buffered. Also typical of dilute general purpose developers, macro contrast decreases with dilution due to controlled exhaustion in the highlights. This can be further managed with changes in agitation. It is an excellent developer, probably the best all around general purpose developer out there at this point when evaluating the balance of tonality, graininess, sharpness and speed in totality.
Perceptol is different. It is an ultra-fine grain developer based on metol and high solvent activity at a relatively low pH. In addition to sodium sulfite it uses sodium chloride for additional solvent action, making it extremely fine grained, but noticeably less sharp at stock strength than standard fine grain developers like XTOL at stock strength. Further, there is a speed loss, usually of around 1 stop compared to most general purpose developers. Dilution of Perceptol increases sharpness and grain (as is the case with any solvent developer, but the differences are smaller due to its initially high solvent action. At 1+3 (my preferred dilution), it has good sharpness, not as sharp as XTOL, but not as grainy either. In addition, being a metol only developer, dilution makes it very effective at controlling high contrast. It is pleasantly soft working.
So in summary, compared with XTOL, Perceptol will exhibit:
-Lower film speed
-Finer grain
-Lower acutance
-Lower contrast
I have two critical (in my humble opinion) recommendations for the best use of Perceptol (or Microdol), both concerning getting minimum grain, which is presumably one of the key reasons for using it. These developers are finicky about speed and contrast. If you want fine grain, you need to accept the speed loss, and the enherently lower contrast nature of the developer:
1. Determine your effective film speed at stock strength (probably around 1 stop lower than normal), and continue to use that EI when diluting 1+1 or 1+3. Usually instructions and guidelines indicate that when diluting a developer, there is an effective film speed increase due mostly to compensating action. So you'll often read that at say 1+3 for Perceptol, you can use your regular speed. I advise you not to do that. Getting anywhere near normal speed in Perceptol, regardless of dilution, amounts to an effective "push" - and it must be noted based on my tests that graininess in Perceptol is exceedingly sensitive to development time. As development times are extended, the extra fine grain properties are quickly lost.
2. Related to (1) above, same goes for contrast. Extrenely fine grain is maintained only when development is to normal (actually slightly soft normal) or reduced contrast. Do not attempt expansions. Grain will increase dramatically and you therefore gain nothing versus using a general purpose developer. I consider the lower contrast nature of Perceptol to be one if its excellent properties, but if you are interested in higher contrast work, XTOL will be just as fine grained, with significantly higher film speed.
Notes on specific films in Perceptol, developed to normal contrast, as compared to Ilford DDX (a general purpose developer):
-TMX 100 is exceedingly fine grained. Even at 1+3 the increase in grain is very small, and graininess is still noticeably finer than in DDX. I would highly recommend TMX in Percepto 1+3 for anyone looking for soft or minus development, pretty good sharpness and extremely fine grain.
-Acros is slightly grainier than TMX, but even at 1+3 still slightly less grainy than DDX
-Delta 100 is slightly graininer than Acros, and at 1+3 graininess is approximately equivalent to DDX
-Pan F+ is similar to Delta 100 in terms of the overall impression of graininess, although the grain structure is different and it looks subjectively sharper to me.