[EricH]

That depends a lot on the wheels. I would even go so far as to say that the tread choice would have more variation. But...

It also depends on the drivetrain. Confining ourselves to tank drivetrains, the single biggest factor is going to be the ratio of trackwidth (the distance between the left and right wheels) to wheelbase (distance between front and back wheels that are in contact with the floor). Bigger is better (unless your robot is falling over due to its own motion) for this ratio, as it relates to turning (but there are other design considerations as well, which I won't go into here as they're game-dependent).

A 6WD flat drivetrain will probably not turn as well as an 8WD flat drivetrain, but for different reasons than a 6WD drop vs. and 8WD drop. In this specific case, the 8WD has a slightly lower frictional force from the far corner wheels (less normal force on those wheels) with which to resist turning. A 4WD tank will be a nuisance to turn smoothly.

But when a dropped center wheel is introduced in a 6WD, the effective wheelbase is shortened. An 8WD has two dropped center wheels on each side, which will further shorten the wheelbase (and tends to remove the rocking motion of a 6WD). The shorter wheelbase improves turning, assuming the trackwidth stays the same.