Just got my D500 and 200-500 in. My initial impressions are very good. I thought I would share some thoughts on how the system compares to Pentax. I will start out by saying that if your goal is to shoot stationary subjects then you can't really go wrong with Pentax gear and they have some great options. There are three factors that led to me switching to Nikon.
1.) AF performance on the D500 is outstanding and much better for tracking moving subjects than any Pentax body. This is the main factor that influenced in my switch.
2.) Future proofing. I'm planning to buy a super-telephoto at some point (probably the Nikon 500mm f/5.6 PF) and the Pentax choices are much more limited. I really hope that Sigma and Tamron will revisit Pentax at some point since that would add more options.
3.) I use an a7 for general purpose photography so any other advantages of Pentax for applications outside of wildlife don't benefit me.
The thing I do miss about the K3II is it's smaller size body. I think it matches a small body size with great ergonomics better than anything out there. The D500 is close ergonomically, but much larger. I miss the light weight of the DA 300 for sure, especially when doing closeups. That was a really special lens. Enough talking, here are some images...