This post goes into excellent detail about non-omni-directional wheels:
http://www.chiefdelphi.com/forums/sh...05&postcount=9 Though what form of traction wheels you use is less important than the kind of drivetrain, since most traction wheels are reasonably interchangable. (All AndyMark wheels use the same mount pattern.)
Mecanum wheels are only used in mecanum drivetrains (
at least for driving).
Simple omni-wheels are used only to reduce scrub forces when turning tank-style drivetrains that also use traction wheels or in holonomic drivetrains where (typically) 4 omniwheels are set up in a square pattern.