I see mecanums as an assist rather than a straight on defense. While it would be tough to use a bot with mecanum wheels to push aside a 6WD robot with treads, the maneuverability of mecanum wheels allow them to play defense in a more indirect sort of way. For example, there was a team at Cal Games yesterday who had a mecum drive, and used their mecanum advantage to heard tubes to their alliance, and to keep them away from their opponents. They executed a very good starvation tactic, and left the other team tubeless. We did a similar tactic at Cal Games, but we had a 6WD.

If you want to play defense, and can play indirectly without any opponents getting in the way, then mecanum is the way to go. If opponents pushing is a problem, then you may want to go with a stronger drive train.