Like Scott I have not compared the Denk 21mm and Pentax XW 20mm with each other. In fact I have never looked through an XW 20mm. However, I do have a pair of Denk 21mm oculars and some different focal length XW eyepieces (<=10mm and 30mm) and I like both. The apparent field of view of the XW is a tiny bit larger (70° versus ~65°). Both series of oculars offer plenty of eye relief, even for those of us who wear spectacles. I guess the best I can offer is that there is probably no bad choice between the two.
As Scott points out there are other options you might want to check out, e.g., the Panoptic 24mm (vignettes a little in the Denk II with a PowerSwitch, check your binoviewer's clear aperture) and the Vixen LVW 22mm to name just two.
Clear skies and happy binoviewing with whatever you choose!