[Oblarg]

One thing to keep in mind is that in the omni-drive, rollers will be turning when you are moving forward or backward. In the mecanum drive, they will not. Whenever the rollers turn, you have non-negligible frictional losses - in fact, if you work out the geometry, you'll see that frictional losses in the spinning of the rollers is the only reason that mecanums strafe slower than their forward/backward movement.