[Alan Anderson]

The two possibilities have very different turning behavior. Imagine the equivalent omniwheel drivebase with the wheels in an X shape instead of a diamond. You simply cannot turn yourself in place using that arrangement, but you also cannot prevent yourself from being turned by an outside force.

If the mecanum rollers don't all point at the same place in the middle of the robot, as would be the case with a rectangular drivebase, you actually do have a slight ability to turn. But switching them to the other orientation will give much better control.