The *istD was the top of the line Pentax. The *istDL is the entry level. I have the *istDS.
The DL has a pentamirror. The DS and D models have a pentaprism. All use the same sensor. There are firmware differences etc. Basically, Pentax has deleted a few features on the *istDS as to make it a medium range camera. The *istDS2 is basically the same as the *istDS but with a larger LCD. All the *st cameras use the same Sony sensor and Pentax shutter.
I have not looked at the K10D, K100D and K110D.
The advantage of the Pentax is the extremely large variety of used lenses. Used screw mount and Kmount lenses are dirt cheap. Also the build quality seems to be better than the entry level Canon.
In all I have been very happy with my *istDS. Of course I have a large collection of Pentax and Pentax mount lenses.