My observation so far...The Ketron is the best (Audio drums)..
The BK5 comes real close and they are midi drums..

The s950 audio drums are decent..except they recorded with too much effects and you have to live with it..another observation,,the Yamaha midi drums sound pretty average (another term for poor) compared to their audio tease..

Still waiting to hear a recording of Ketron midi drums..( although I already heard them and believe the BK midi drums out shine the midi drums on the Ketron too)

I recall listening to audio loops in the Audya and substituting them with midi sounds...TALK about being naked..and this is within the same instrument..

The original comment by Diki seems fairly accurate...the BK is pretty close (not better) than audio drums on the Ketron...but the difference is small..and edits are plentiful on the BK..

Yamaha still needs to upgrade the midi drum kits...and audio drums seems to be a tease presently, with a test for future interest..

BTW: They all sound great compared to a decade ago..