[Alex.Norton]

The difference is in whether or not they're constrained and allowed to move forward by the motion of the bot. In a mecanum if it is moving forward the rollers won't turn because if they were the wheel would have a lateral velocity (different from force, there is a lateral force). On an omni when moving in the forward direction there is a lateral velocity so the rollers do turn.

However this means that in an ideal situation, when you have no friction in the bearings you will not see ANY difference between the two. You may see a slight difference in top speed because of the angle on the wheels but you will see absolutly no difference in power since no power is used up in heating the bearings.

In the real world you will see that in the forward direction since the mecanum rollers aren't turning there will be a slightly better efficiency and more power will get to the floor, but I would be willing to bet that you couldn't measure the difference cause when your talking about a 321 watt motor, the amount lost to the bearings is inconsiquecial.