*ist DS, *ist DL, *ist DS2, *ist DL2, K100D, K110D and K100D Super are basically the same hardware platform with minor variations, but they all share the same 6MP sensor so expect identical image quality from all of them.
K100D adds shake reduction, K100D Super also support for SDM lenses.
I have *ist DS and I am not willing to part with it. It is very simple camera to use, small and easy to carry around, and with great image quality. Compared to the latest bodies, AF is slower, there is no wireless flash support, color LCD screen is smaller, slowish continuous FPS rate (but I never use it), no shake reduction (although it is added to K100D).
If you can find one lightly used, or any from 6MP family, get it. Perhaps avoid *ist DL/DL2 (no AF point indicators and a few other corners cut).
Just recalled, you can go here and check the differences:
Pentax Cameras | Pentax K-5 vs. Pentax K-r vs. Pentax K-x - Pentax DSLR Comparison - PentaxForums.com