As someone who owns a D810 and D800, owned a huge collection of Pentax kit, and a Fuji XT1 kit (along with playing with Olympus and Sony as well...all 2013-2015 offerings) now own only Nikon and film Pentax. I am also looking at getting Fuji again... see my other topic on why. and it has very little, if nothing to do with image quality. Pentax, Nikon, and Fuji all help make beautiful images. In fact, I made custom profiles for my Nikon RAW's to get close to my Fuji files and then adjusted to taste.
The reasons I see to take Fuji over Nikon/Pentax full frame is because of the portability, size, and weight. When it comes to image quality they are different, but very similar and just depend on what you enjoy shooting more.
I lost my love for Pentax after getting into Nikon. The D810 is the best DSLR I have ever owned or used. Period. The XT1 however is the most enjoyable camera I have ever used.
No matter what I do during the remainder of 2017 and 2018 I will be buying an XT2 and 2-4 lenses. I will no longer invest in Nikon or Pentax at this time. Mirrorless is the future. I will hold out until January maybe to see if any of the rumors of a full frame Fuji/NIkon/Pentax mirrorless get any stronger before making a decision... but the XT2 is firmly in my sights right now and has been for a while. I'm mainly a wedding and portrait photographer too.