The differences between Nissin Di866 and Di866 Mark II include:

Mount:
Mark I has a plastic mount while mark II has a metal mount.

Sound of zooming:
Mark II zooming of the flash head is softer than the Mark I.

Sub-flash power.
Mark II sub-flash power is able to go to 1/32 while the Mark I's can go to 1/16.

Viewing angle of coloured LCD panel
Mark II's viewing angle is wider than Mark I's