I have both lens.
However, I am going to sell the Nikon AF-S 80-400 vr since I am going to shift back to Pentax's FF.
The photos for reference below is from Nikon D810 with Nikon AF-S 80-400 vr
http://www.dchome.net/forum.php?mod=viewthread&tid=1280472
The photos for reference below is from Pentax K-3 with Pentax DFA 150-450mm
http://www.dchome.net/forum.php?mod=viewthread&tid=1296769&extra=page%3D1
My conclusion is the AFC from Nikon is much better than Pentax, whilst AFS performance seem to be the same level from both lens. Although I did try the AFC of the Pentax's combination, however, I did not post any photos mainly due to the result was not up to my standard and satisfaction.
The advantage of the Pentax's lens is AW, whist Nikon's is not weather sealed.
Regarding the image quality, I think the Nikon's lens is a little bit better in the aspects of CA.
Concerning about sharpness, I think both lens are great.
But of course, the statements above are based on my own impressions not being tested with scientific method.