Hey all,
I'm stealing from my own thread (see below), but I think the Pentax K7, by inherentance from K20D, will probably have higher frequency detail than 5D/5DMKII, but I think the Canon will probably beat K7's chroma and sensor noise, albeit with loss of some detail.
I've been pondering the K7 vs. 5DMK2 vs. D700 as well and spent quite a bit of time looking at pictures from all the cameras. The stunning pictures are *amazing* from all three cameras, but the bad pictures from the K20D have a "cheap" look to them -- noise(chroma) and color gradations that remind me of a bad point & shoot.
I think the largest advantage Pentax has is their ability to use older lenses without having to modify the camera (M42, Pentax-K mount) followed by the resilient weather sealing. The Canon crowd (5D, 5DMK2) has resorted to shaving the mirror in order to get older lenses to fit. Once it's done however, the Canon results with Taks and Zeiss's are outstanding.
But do you want to void your warranty and potentially suffer from weather-related failures? Or do you want to deal with a crop-sensor and the need to further post-process your images to more closely match noise out of a Canon/Nikon? If you can't tell, I'm still on the fence.
I think feature-wise Nikon has Canon beat except for the 12MP vs. 21MP. Although I personally would prefer Canon over Nikon, if Nikon had a higher resolution sensor and was more cost competitive, the 5DMK2 would be out of the running.
K20D vs the Canon 5D & 5D MK II:
K20D:
http://a.img-dpreview.com/reviews/PentaxK20D/Samples/Comparedto/pentaxK20D_ISO100-acr.jpg
5D:
http://a.img-dpreview.com/reviews/CanonEOS5DMarkII/Samples/Compareto/Studio/...00-raw-acr.jpg
5DMK2:
http://a.img-dpreview.com/reviews/CanonEOS5DMarkII/Samples/Compareto/Studio/...ISO100-ACR.JPG
Two things I noticed, the K20D has Canon beat in-terms of wall detail and sharpness on the Queen of Hearts card. I think the K20D might have a weaker AA filter, since both Canons should have equal or better resolving power.
A strike against the K20D if you look at the edge of the Martini bottle in the Pentax shot you can see what looks like CA or a second shadow on the bottle -- the effect is not as pronounced in the Canon shots. I'm wondering out aloud if that's a lens limitation or sensor/camera issue though.
Anyway, those are my stream of consciousness random thoughts on the subject...