I have decided that this is the main difference in archetypal terms -
Type 8 is the authoritarian Boss type; type 6 is the anti-authoritarian Rebel type.
According to that page (and Jennifer explained this also), the Type 6 has problems with controlling impulses, the type 8 on the other hand is in control of him or herself at all times. The type 8 does not lash out wildly. The type 8 does not force-choke anybody out of impulse. The type 8 has a plan and sticks to it, even if that plan is murderous. The type 6, at that level, has lost control.