Originally posted by rawr A while ago I had to clean up several hundred high-ISO 5D2 shots on behalf of a friend who commissioned another photog to do a bunch of portraits of orchestra musicians in a variety of dim, rustic interiors.
Based on that experience, I believe that the K-5 is capable of performing as well, if not better, than the 5D2 at high ISO. It's not just the SNR of the K-5 vs the 5D2, it's also the lack of DR in the 5D2, and the horrid banding noise of the 5D2 in any shadow at ISO past about 1600 that destroys detail and is a pain to clean up. I can't post any of those 5D2 examples since they aren't mine to post, unfortunately.
I recently shot a swimming event where I was up to 12,800 ISO on my K5 due to the terrible lighting and slow tele lens. I had my 5DII with me as well with the 24-105 (one of my fav lenses) for non-tele shots. I also had that ISO cranked up pretty high (I think 6400?) I found that I somewhat preferred the noise on the K5 images, even though I think the 5DII is supposed to beat it at high ISO.
There was a pro freelancer there with a 5DIII and the 70-200 f/2.8. He had just upgraded from the 5DII and said that the 5DIII was a huge upgrade for his needs (mostly due to the AF) and he was very happy with it.
I do still prefer the 5DII for my landscapes due to the extra bit of res and sharpness the 5DII gives, but I am ready to trade it out for the D600 at this point. If I were shooting sports or something, I'd have to give another look to the 5DIII, but for my needs, Nikon is better.
Keep in mind that I judge this on first hand experience with the 5DII and K5 Classic, and armchair reading about the pros and cons of the D600 and 5DIII.