[EricH]

Speed and traction: Depends mainly on surfaces and gearing. A 6WD can have the same speed as an 8WD, or faster or slower, based solely on gearing.

Why do some wheels get dropped lower? If you've ever tried a 4WD non-omni, long configuration on carpet, you may have noticed that they bounce whenever they turn. This is due to the long wheelbase and relatively high traction. 6WD and 8WD have the same problem unless the center wheels are dropped, effectively shortening the wheelbase.

6WD with omnis on the corners vs 6WD with traction on the corners: Use caution. Personally, I'd go with high traction in the center and lower-traction (possibly omni) on the corners. If you're doing omnis, you can do a flat drive instead of a drop-center drive. If you're using traction, you'll want the drop.

8WD: Anything I've said about 6WD applies to the 8WD as well, except that the 8WD can be a little more forgiving in handling and dealing with obstacles and a little less forgiving when building, depending on exact type.

The dropped center is not just for the terrain. One of the most-copied drivetrains is the West Coast Drive, a 6WD drop-center cantilevered live-axle pioneered by teams 254 and 60 in 2004. As explained above, the drop is necessary for smooth turning.