Ask that question in a brand-specific forum & what did you expect to get in response? Let me just pipe in with a few things. The build quality and sealing of the K-5/II series is much more robust than the D600. I would trust the K-5 II in wet conditions, whereas I would not even subject my D600 to the same conditions. I own zilch in terms of Pentax gear, but everything I've read from owners since the release of the K-5 has swayed me into believing it, along with the K-5 II, is as robust as the Oly E-3 or E-5, the former of which I used since it was first released. I'm kind of in the opposite position as yourself because I own a D600 and am looking at getting the K-5 II for the weathersealing. I can tell you right now that the kit lens, 24-85VR, is a fantastic little lens. There are a number of online comparisons between it and the 24-70 2.8 from Nikon (non-IS), along with the Tamron 24-70VC 2.8. It competes quite well with both. It won't trounce the Nikon 24-70 2.8, but it does have VC. A little softer in the corners at some focal lengths, but also similar for the most part in the center.
I personally like big, chunky camera bodies. I was highly impressed when I got the D600 because it is both smaller and lighter than my old Oly E-3, which was unexpected & nice. From what I've heard, the K-5/II/IIs grip is great. If you want a better visual as to the size difference between the two, have a look here:
Compare camera dimensions side by side
Yes, there have been a number of people posting all over about the oil spots/dust on the D600 sensors. Mine was dirty as well, and I would guess I spent about 3 hours of time & about $100 worth of various products (electric blower, sensor pen, swabs, etc.) for it. Mine is now clean. Not sure how long it will remain so since one of the theories floating around is that the oil spots are the result of oil splattering off the shutter mechanism. Some have claimed it will go away in a couple thousand shutter releases, but who knows. I know I can clean it myself, so it isn't a huge dramatic deal.
Even though I own nothing Pentax, I can say that I have a few thousand RAW files that I have downloaded from various sites from the K-5, K-5 II and IIs. The D600 IQ is just superb in comparison, and extremely malleable. I'm positive others here will disagree. To answer your question, if I absolutely needed weathersealing, I'd go for Pentax. If weathersealing were not important, then the D600 by far. One of the guys on DPR was arguing with me that the D600, which has the same sealing as the D800, is just as weathersealed as the K-5 II. I do not believe him whatsoever, and the argument fizzled out because I was not about to subject my D600 to the same conditions as I have my Oly E-3. That would be an expensive test, especially if I am right.
Lenses are also a consideration, or the ONLY consideration depending on what kind of shooter you are & how many lenses you will need. I'm a zoom kind of guy myself. If I am on the side of a ledge out backpacking in the desert, I'd like to be able to zoom in or out as needed. A prime just would not be beneficial to my needs, although I did pick up the cheap 50mm Nikkor for fun. There are a lot of zoom lenses to choose from, and some of them can be quite expensive if you absolutely need the widest aperture possible. But, there are also quite a few 3rd party lenses that do 95% of the same for much less. As for myself & Pentax, I will only be buying the 18-135WR as the sole lens for the K-5 II or IIs. I simply do not trust any of the SDM lenses, and my whole point of buying into Pentax would be for the weather sealing.
Best of luck in your decision.