I have used both for a long time and I can confirm that they are the same in sharpness and brightness.
I have owned 3 DS units and 2 D ones, the levelling of the DS models are better but the coverage of the D models are more accurate for all of them, if you compared these aspects. Particularly, the DS finder's alignment against the CCD is slightly off for all units I have but then the levelling of the D units were slightly off then.
If you need a bright finder, the K10D's one is improved. I can't comment on the centering and levelling accuracy as I haven't checked these.
Do note also that the trade-off for a brighter finder is that you might experience overexposure issue when you used older non-A Pentax lenses for stopped down metering (owing to a newly design more transparent focusing screen):-
RiceHigh's Pentax Blog: When the Focusing Screen Lies (for Focusing and Metering)
Hope these help.