From someone who went that route... I can save you some trouble.
When the D600 first came out I picked it up because I was enamored with full frame. I wanted full-frame badly and thought it was the next logical step for me. I was a hobbyist who did some odd jobs here and there, but still mostly shooting urban and landscape work. I honestly didn't find a big enough difference in quality between my D600 and my K5. Granted I was using FA Limited and DA* lenses vs the kit lens, 24mm f2.8 D, 50mm f1.8 G, 85mm f1.8 G, 80-200mm f2.8 D on the Nikon (only the 85 I still own), but I didn't feel like it was worth the investment for what I was doing vs what I was getting from my K5. I felt like I was hauling a lot of extra weight and bulk for no reason.
Then I started doing more portrait sessions and I started enjoying the transition of the DOF fall off more of the full frame system, not to mention the better flash capabilities. (The SB700/SB910 are two of the very best hot-shoe flashes I've ever used! Especially when using the CLS system). So I sold the D600, 24 2.8, 50 1.8, 80-200 f2.8 for a D800 and a 24-70mm f2.8 G. The D800 is MUCH closer to Pentax ergonomics than the D600 (which was a huge pain for me) and I can switch fluidly between my D800/D810/K3/K5 with ease. But my shooting style has dramatically changed and I'm now a part-time photographer who shoots very little landscape and urban work anymore. All my work is portraits now either paid or TFP.
For me now it's about selecting the right tool for the right job. For strobist work the D810 cannot be beat by Pentax, especially on location and outdoors. In studio the K3 can hold it's own right along side it.
For landscape work... I don't know what I would do if I was comparing say the D610/D750/K3ii. On one hand, the D750 with Tokina 17-35mm f4.0 (a lens I own and shoot a lot with my D800) would be a really amazing kit. But, it's big and it's bulky. Even the Sigma 24mm f1.4 ART would be epic. Or I could just go out there with a Pentax K3ii with the DA 15mm f4 Limited and have almost no weight in my bag and very limited bulk but still take amazing images, not to mention have the GPS astrotracer for astro photography....
All I know is that I only bring the Nikon out for my studio or on location strobist work now. It's killing my body hauling that kit around. (My Nikon kit backpack is 10kg - D810, 24-70mm f2.8, 70-200mm f2.8, 50mm art 1.4, 135mm f2.0 DC and Profoto B1 strobe).
---------- Post added 12-08-2015 at 11:45 AM ----------
for fun.... without looking at the EXIF or Metadata.... can you match the image to the camera? K3, K5, D600, D800, D810, XT-1, Q7
A:
B:
C:
D:
E:
F:
G: