Tank drive is the way to go, as a rookie team last year we can attest that it did wonders for us.
a) It is easy to assemble. Just attach the drill motor assemblies to a frame, add a sprocket and a few chains... voila!
b) It provides both power and precise control. Lets face it... CASTERS SUCK! How many teams last year with casters didn't have trouble making it up the ramp? Not many
c) Easy to add upgraded functionality. We just happened to add a servo gear shifter so we could be fast, or powerful.
Here is a pic of what our base looked like last year. Very simple square base with a two beams running in the center to hold the drills.