I think your practical experience confirms my technical evaluation of voice
recorders (which I haven't used). It is only in the last year or so that
these recorders have started to appear with much higher sampling rates.
As far as I can tell the only difference between the DM-10, DM-20 and
(new)DS-2200 is in the memory. DM-20, as you note, has more internal memory
that the DM-10, and the DS-2200 uses removable xD-picture cards.