Our team just finished discussing this problem today. We went with 6WD. Why?
Motor Consumption.
There was talk about Maneuverability and Torque and Strafing and Speed. But here's what it came down to:
We wanted our arm to be able to get into whatever position we wanted as fast as possible.
Now, speed is irrelevant because whether you have two CIMs or 4 CIMs running your drivetrain, you will go the same speed. Obviously.
Maneuverability IS what you are losing here. But consider this. You can teach a driver how to be more accurate, with more practice comes greater accuracy. No matter how much the robot practices, it will not get faster. Plus, the built "Safe Zones" in this game are so frequent it's like you're playing match mode on "Easy". If you can get into the safe zone...does it matter? Also, when you're positioning a tube in close range, does it really matter with no defense which is faster?
Torque really doesn't matter much this year (IMO, and for our strategy), so more motors aren't a necessity.
Now teams operate in different ways so there really isn't one answer for this. Our team has never done a mechanum drive before. As easy as it may be, we can build a 6WD with our eyes shut. Also, this year's game is screaming "Mechanum/Crab/Swerve". Might it be easier to outmaneuver a sea of multi-directional drivetrains with something you're more familiar with? In 2007 we had a problem with our arm motor and had to use the big CIM (thank god it was available) to compensate. I think you really need to prioritize what you REALLY want to accomplish in this game and go for that first.
There are VERY few teams that can play "God-Bot"