I've used both and prefer the 540. Metz user interface is too menu-driven, Pentax controls are easier IMO. Plus used off-camera, the Pentax was more reliable due to its better sensor location (I believe this has been corrected on the new Metz, 58 version 2.) Another problem with the Metz was that it could not be used as controller/master in P-TTL, to operate an off-camera P-TTL slave; it could only be used as the slave- so you'd still have to use either a Pentax 360 or 540 on-camera to power your Metz as slave in P-TTL. That review in the link that was referred to, in the earlier post, does not mention these issues...
This and the HSS issue have been discussed in other threads, do a search because the Metz vs Pentax question has been brought up
many times. FWIW,
if I was only ever going to use the flash on-camera, or on a bracket, I'd go with the Metz, but if you need multi-flash off-camera capabilities I personally prefer the Pentax 540.
P.S. Every Pentax AF540 owner should do themselves a favor and check the adjustment on the trimpots, it really helps.
https://www.pentaxforums.com/forums/pentax-camera-field-accessories/103779-in...x-flashes.html