[Peck]

The moment of inertia for the robot is a complex system. (Assuming from here a simple normal system) A low moment of inertia results in a lower rotational momentum and therefor a greater ability to start and stop turning. A high moment of inertia results in a higher rotational momentum and therefor a greater desire to keep turning at the same rate.

If you only want to have a consistent, smooth turn, a high moment of inertia is what you want BUT at the cost of responsiveness and control.

also keep in mind that your CG is only ideally placed when centered if your robot turns exactly around the CG.

Finally: the choppy turning may be because of your programing or your power transfer method.