Thanks for very good images! I also downloaded the raw images and spent some time pixel peeping. Today I also tested Iridient Raw Developer.
What I see is:
1) The colors are very different. The same difference in color is seen in Raw developer.
2) Nikon greens contain much more yellow. It may be that Nikon has better Yellow Green separation. This is not obvious to me from DxO-mark data. I don't know if Pentax or Nikon is better but they are quite different.
3) Resolution on Pentax is significantly better, as expected
4) The Pentax lens seems to be very sharp across the field. Let's not forget that the 645D is a cropped format camera so the sweet spot of the lens may be used for the entire picture. Anyway the combo is very good.
5) I don't see a lot of difference between the two in shadow detail. In my view the DR of both cameras exceeds the DR of the subject.
I'm impressed by the P645D. Regarding cost it needs to be kept in mind that it's camera and lenses. For someone having a bunch of excellent 645 lenses and demanding the uttermost image quality the P645D is a no brainer.
On the other hand the premium cost over a Canon 5DII or a Sony Alpha 850/900 is quite hefty.
I've done some more testing outdoors with my 645D vs Nikon D3x
Daylight landscape, ISO 100 RAW
D3x, 24-70 zoom lens @ f8 live view manual focus
645D, 45-85mm zoom lens @ f11 autofocus
The focus point was the tree on the center of the image.
After capture daylight color balance in LR, no sharpening.
Sharpening in CS5 D3x shot USM 300,0.4,1 645D USM 300,0.4,1
Some adjustment in CS5 to match the color.
ACR does a poor job with D3x color as rumored.
This was shot near Angels Camp, CA in the Sierra foothills and it really was very, very green.
Warning big files D3x is 44MB, 645D file is 77MB
Comments of course are welcome.