Until the M92 runs Android, I guess a main benefit of Kindle DX be the syncing of bookmarks, notes, highlights and read status between Kindle devices and apps (allthough not as much for sideloaded/"personal"/non-Amazon content).
As for Onyx, it seems to be much better for PDFs and annotations (but not when it comes to exporting/syncing). I've never tried an Onyx device myself yet, as I'm still considering tablet vs eInk for that need.
How Onyx handles epubs, picture zooming and other issues I don't know.
(Trying to figure stuff out just by reading forums is hard, but unfortunately most stores as far away ...