Originally posted by Kasuras I had not taken the in body image stabilization into account, that's a good point.
The 35/1.8 is indeed much cheaper than the Pentax 31/1.8. On the other hand,I have heard good things about the Sigma 35/1.4 and 30/1.4 and especially the latter is substantially cheaper. A little more expensive than the Nikon 35/1.8 but that's something I can overcome.
A difficult decision, and I haven't really made a choice yet.
Trade-offs !
I looked at your links to your photo's and you aren't shooting sports. If you were shooting sports you will need the slightly faster auto focus of the D7000
WITH a VR lens. By the sounds of it you just need better low light auto focus.
Anyway the K-5 renders an image better: produces a better 3D effect; better coloration; it has sensor based stabilization; operates down to -10 degrees Celsius; better weather sealing rated to a monsoonal downpour (I spoke to a Nikonian working at a store who lost his $7000-8000 D3 to water getting in. Despite that, he bought a D800 & D800E - gotta admire that dedication. I walked away rolling my eye's ! lol).
I've used non VR lenses on a Nikon and its horrible, I'll never buy another camera without a stabilized sensor - its like the stone age ! (This is my personal experience, I don't know about other peoples experiences, perhaps they got by okay with high ISO's). But me, NEVER AGAIN ! (I have K-5 and Sony A65)
When I bought my K-5, for fun I asked the sales person how it compares in its class, and she said the K-5 outperforms the 7D in high ISO, and mentioned the D7000's sensor pattern has a problem with rendering green - like green grass - not sure if it still has that problem or if it matters to you when shooting in black and white.
As for Lenses:
The Nikon 35mm DX f/1.8 is very low in resolution according to Photozones test - an MTF peak center resolution of 2173 @f/1.8 and @10megapixels. Even if you go with the Nikon D7000, your best bet is to go Sigma, either the 30mm f/1,4 (Photozone MTF peak center 2453), or the new Sigma-A 35mm 1.4 which "Lenstip" claims is the best 35mm and out resolves everything in its class that's twice the price!
If you go with a K-5, the Pentax FA 31 f/1.8 - according Photozone - has an MTF peak center resolution of 2345@f/4 @10megapixels, and is claimed by many to be one of the best Pentax lenses. But at over $1000, it aint cheap. The Pentax DA 35 f/2 has a peak center resolution of 2336 @f/4 @ 10megapixels. The Sigma models above are also a viable alternative or even the Pentax FA 43mm f/1.9 is the highest resolution Pentax prime (according to the data on Photozone).
In a nutshell, these are some lenses for your consideration, I would suggest you take a look at the Pentax Forums user reviews of these lenses to get an idea of their characteristics.
Also checkout Photozone and Lenstip's descriptions of those lenses characteristics also. I personally avoid DXO's lens reviews or any other source that do not publish MTF data - so's I can dissect it ! lol
That being said, Photozone and Lenstip have a clear and obvious bias toward Canon and Nikon. I spent months analyzing/cataloging and tabling Photozones tests - for my own interest. In all honesty there's something dodgy about a few things. The first I discovered is that if a third party lens out resolves a legendary Canon lens, it isn't published in the full frame tests directly against a legendary lens - similar with the Nikon tests. They give glowing remarks mostly to Canon and Nikon. The Nikon 10 and 16 megapixel tests do not correlate/concur with the percentile increase in border to center resolution. In other words, there is a massive difference between center and border resolution in 16 mp tests, and that quantified as a percentage is no where near similar to the quantified percentage between center and borders in the 10 mp tests. Its as if there is a hole in the middle of the anti-aliasing filter in Nikon 16mp tests! This has been done because Pentax annihilated Nikon in the 10mp tests. Cross check resolution at DP review or Image Resources image samples - Nikon 85mm and Pentax 50mm macro (similar MTF) - K-5 out resolves Nikon D7000 IMO. So this is an honest warning about Photozones tests, believe me I spent months analyzing them without bias. This research also caused me to go Pentax.
Go to a shop, spend time testing out D7000 and K-5 try the lenses you're interested in and compare.
Just remember: If you go Pentax you'll have stabilization on fish-eye, wide-angle and prime lenses ! Check and mate !
Good luck with it !