K20D is a pretty old camera, and even back in its heyday it was worse than the Canon/Nikon counterparts at high ISO.
If you shoot at 800 or 1600 ISO more than occassionally, I would get the K-X. Sure, build quality is important, but keep in mind that a weather-sealed body means very little if you don't have a weather-sealed lens to complete the sealing on the body.
I'm not so sure there, especially with saying that K-20 back worse than its Canon/Nikon counterparts at high iso. I have a K100 and a K20, and I prefer the K-20 at all iso levels. I've never seen it noticably worse than the competition. Slower? Yes. But Worse IQ? Depends what you want and what you look for because I think its more subjective, especially when you are comparing to cameras during its time.
For me the K-20 images appeared slightly softer compared to the K-x, but I attribute part of that to the fact that it demands more out of a lens in terms of resolving power (14 MP sensor vs 6 MP sensor). You sharpen up images anyways.
I think a weather sealed system is a much more powerful argument with pentax now vs a while back because there are two entry level lenses at entry level prices. If I was going Pentax, I can't imagine not wanting to take advantage of weather sealing.
Imo I'm glad you went with a K-20. I can't ever imagine going back to a consumer slr in terms of controls. Its clearly inferior and I love accessing everything using buttons. My friends has a K-x that I use, and while I like the high iso (which is better than the K20), I just can't part with all the buttons and two wheels.