Personally, I like 8WD, although my opinion might be a bit biased because I haven't driven a 6WD robot anywhere near as much as I've driven an 8 wheeled one.
This year my team used eight 4" diameter wheels geared 7:1 and I loved it.
Here is a video of us playing with the chassis a week into build season:
http://www.youtube.com/watch?v=inI40uMKuEE
If I remember correctly, the center two wheels are dropped 1/16". We later added another two motors to the drive train which gave us a nice balance of speed and power.