[David]

1. "Rocker Style"
Pros:
Decent Turning (Acts Like 4wd)
Great Traction
Designed Into Kitbot Frame
Good for climbing
Cons:
As the name suggests, if you do not balance it properly, it will rock back and forth constantly.
(I believe Beach Bots 2007 bot are a good example of a well balanced one)

2. Omnis
Pros:
Great Turning (when you want to)
Good traction
Stable (doesn't rock)
Cons:
Easily turned (when opponents want to)
Not as good as "Rocker Style" at climbing

A third type is having 6 narrow traction wheels all touching. This has about the same pros and cons as "rocker style", but it does not rock (Children of the Swamp Designed a nice a year or so ago, and I think Cybersonics use it)