[CalTran]

This is my first year on Team 2410 and we used a chain-based mecanum drive. From what I saw, here's some specs on it:
Pros:
Easy to set up
Easy to fix
Gave us a lot of experimentation room as far as where the gear boxes went
More space for the edges of our robot

Cons:
Chain is like a woman. High maintenance XD
Chain would get caught on various things
Needed grease every so often
Sometimes would fall off if not tuned exactly

So that's what I picked up on and could pull off the top of my head. I don't believe we ever tried a direct drive mecanum so I can't help on that. Hope my 2cents helps.