My post from a similar thread:
https://www.pentaxforums.com/forums/pentax-dslr-discussion/123861-refurb-k20d-vs-new-k-x.html
I have the K20D and K-x.
- I prefer the K20D image quality at any ISO up to 1600. The colour depth is better, it exposes more accurately (K-x burns highlights in matrix mode), and the K-x is more subject to moire.
- The K-x had about 2/3 stop better high ISO performance in my test (tested in raw with NR off).
- K-x in general has more accurate AWB, but goes too blue in daylight. I shoot raw, so AWB doesn't matter.
- The extra modes, switches and second wheel on the K20D make it much nicer to use.
- Added bulk and grip are advantageous with large or heavy lenses and flash. The K-x small size is nice if I'm trying to pack light.
- I notice a focus difference in low light AF, in favour of the K-x. In normal light I see no difference.
- I rarely shoot on continuous high speed and I don't use video, but if you do, both are K-x advantages.
- I never use any focus point other than center, so that K-x disadvantage means nothing to me
- All the reviews say K-x has one stop better dynamic range, but in practice I haven't seen a difference. That's probably because I'm mostly concerned with burned highlights and the DR advantage is on the dark end. The left hand side of the histogram is inconsequential to me 99.9% of the time.
- Having WR is a comfort. I have yet to buy a WR lens, but I will soon.
Here's a comparison I did to illustrate IQ at ISO 200 (shot in Portrait jpeg mode, my default):
Picasa Web Albums - Dan Bonhomme - K-x K20D
Portrait is my default image tone because I find it just right for general use, Bright is too punchy and Natural is too bland. I adjust as necessary in p-p, but the test above is out-of-camera.