I would wait for the possible mid September announcements for Photokina. I would then try to rent the camera I'm after to see if suits me. But at this time, in my opinion Canon 6d is excellent value for money (and its central AF point sensitive down to -3EV is awesome). If you keep your 6d, we cannot speak of a switch but of a complement. Get rid of 5d mk2. It's not worth using it. Buy a D810 (incredible AF, excellent ergonomics, reduced vibration due to the electronic first-curtain shutter and redesigned mirror mechanism, classy shutter sound, amazing DR and awesome 36 MP sharpness and clarity), 2-3 primes (Nikon 28mm f1.8G and Nikon 85mm f1,8G and maybe Nikon 50mm f1.8G if you think a normal lens is a must have for your shooting style) and be prepared to be amazed! Oh... just to add that even a zoom like Nikon 24-120mm f4 paired with the D810 will give you images far better than you would think. Any decent lens used in combination with the D810 will produce better photos than the photos taken with the same lenses but mounted on any other FF camera.
I did it...Bought an 810.
The dealer didn't have any lenses to sell me, but he gave me a 10 year old 35-135mm 3.5-4.5 for free. I played with that for a couple of days and was amazed at the quality of the images that this camera produced with a crappy old lens.....I know that I should have waited for the middle of the month, but I had reached the point with the 5d2 that I didn't even want to look at it. I also have my 6D in for repair due to soft images after 3 months of owning it....so I couldn't wait.
I went to another dealer and bought a better lens for it, the 18-35mm and am quite pleased. In fact the results are so stunning, I don't see any reason to keep any of my canon gear. I will be purchasing a 24-70mm 2.8, and the 70-200mm 2.8 with vrii. Will save up for the 14-24mm and that will be all i need aside from a zeiss prime down the road.
I will sell all of the canon gear to make the switch complete....I waited too long for Canon to get their s*#t together, and loyalty only take you so far.
Thanks for everyones input
All the best,
Axel