[EricH]

6WDs are prone to rocking due to their dropped center wheel (barring other arrangements like omnis or odd CG arrangements). 8WDs are much less prone to rocking because both center wheels are dropped.

The "more traction" argument is one that will depend heavily on the specific drive systems involved. I suspect that some 6WDs have more traction than some 8WDs, as well as the other way around. Think about it this way: The same weight is being spread across more points of contact with the ground, reducing the normal force. OTOH, that's also more points where power can be applied and frictional effects that appear to raise the CoF can happen. The only way to make that claim definitively is to do a full analysis of two systems that are identical other than number of wheels (and associated power transmission).