Some things I've found on my way asking the same questions you do:
http://www.chiefdelphi.com/forums/sh...&highlight=WCD
http://www.chiefdelphi.com/forums/sh...&highlight=WCD
http://www.chiefdelphi.com/forums/sh...&highlight=WCD
http://www.chiefdelphi.com/forums/sh...&highlight=WCD
http://www.chiefdelphi.com/forums/sh...&highlight=WCD
http://www.chiefdelphi.com/forums/sh...&highlight=WCD
http://www.chiefdelphi.com/forums/sh...&highlight=WCD

All in all, WCDs are lighter, allow for direct drive (more efficient), and the wheels a bit farther out for better turning. 6WD vs. 8WD is mainly based off of whether or not the robot has to traverse an object (climbing over the bumps in '12 and '10). 8WD also allows a smaller wheelbase, so that a robot can be more maneuverable, but the turning difference is often nothing a 6WD can't do, and the extra weight isn't worth the little bit of turning help on a flat field.