[Nemo]

I would suggest waiting until the game comes out before you decide whether you want to do a mecanum drive. Some games are going to be better suited than others for that drive. In my opinion, 2011 was a pretty good year to use mecanum, and 2012 was less suited for it.

2011: flat field, pretty big protected offensive zone where defenders can't touch you, scoring pegs are easier to score on if you can strafe, human players can throw the game pieces most or all of the way to the scoring zone.

2012: Harder to balance on bridges with mecanum, strafing less of a benefit for lining up to goals (but admittedly useful for gathering balls). Mecanums can get over the bump, but I don't feel warm and fuzzy about ramming that type of wheel into a steel barrier (seems like rollers would get bent and start to stick)

To address your question, though, here are my thoughts on 6" vs 8" wheels:

6": cheaper, lighter, smaller, less gearing required
8": tougher (except now there is a heavy duty version of the 6" wheels)

Even though we've used 8" twice in the past, I think we'd start by looking at 6" if we were going to switch back to mecanum for some reason. I'd probably be looking at direct driving them with a set of Toughbox Nano gearboxes with one of the lower optional gear ratios.