4 Wheels vs 6 for games like 2014

[jkelleyrtp]

From the last CD threads on this topic, it seems as if the responses were a bit dated so I figured I'd ask the question again, with the idea that in 2016 we might have similar field topography to that of 2014.
With equal weight (normal force), why do teams choose:

• 6wd pure tank
• 6wd drop center (WCD)
• 8wd

...when their resistivity to movement and therefore traction are all the same? If the point of WCD is to emulate a four wheel movement system, why don't teams simply resolve to using four wheels in contact games like 2014? I understand that motor performance changes (or it used to in 2005, according to the old CD threads), but considering the current wheels we have like colsons, what really is the difference? Some of the older responses said that 4wd drives straighter but has a greater impact on current draw than a distributed 6wd.
And to clarify, my idea of a 4wd is a gearbox chained or belted to the two driving wheels, not individually ran wheels like mechanum.