The MK II has greater resolution, bass and extension in the highs than the MK I. It also has a few software problems which have been resolved in the latest chip.
The MK I plays all CDR's that I have where the MK II does not. The MK I is extremely reliable with no software issues whatsoever. While not being as good looking or as sturdy as the MK II, it is still the best sounding CD player I have heard other than the MK II.
As the largest dealer for Audio Aero, I have only had two MK II units, out of all that have shipped, that had physical problems. Both have been taken care of quite well by the distributor.
There is no wrong decision.