They are quite different headphones and both have their flaws.
Bass - Both very well extended but H6 has far more quantity
Midbass- The H6 has recessed midbass- which is it's main flaw IMHO. Due to this,the PM3 will likely sound fuller, particularly with voices where the H6 can sound a tad thin.
Mids - Both very nice and present, the PM3s are probably a pinch more present
Treble - The H6s are well extended but not harsh at all, while the PM3s are heavily rolled off which make them come off as dull sounding
Soundstage - The H6 destroys the PM3 in this department, the PM3 soundstage leaves a lot to be desired, even for a closed headphone
Build - Both are built extremely well but the H6 does use real leather while the PM3 uses a very nice but synthetic leather
Comfort - I never bought the PM3 so not sure how they are for long periods but during my demo I found them very comfortable but will probably sweat a bit. The H6 are the most comfortable headphones I've ever worn.
The planar technology does give an overall tighter sound on the PM3s but for me, this is ruined by the lack of treble extension and soundstage.
Hope this helps!