Unity Multiplayer Performance implications and general differences of TransformSyncMode

The NetworkTransform lets you set the TransformSyncMode.

http://docs.unity3d.com/ScriptReference/Networking.NetworkTransform.TransformSyncMode.html

But it's not clear in the docs what all the different values of the TransformSyncMode actually mean. What's the different between SyncTransform, and RigidBody3d? Why would you use one over the other?