RB67 is a totally mechanical camera. No electronics of any kind, except if you get an optional metered viewfinder. The RZ67 has electronic shutter control, but it can use almost all the lenses from the RB67, in addition to the RZ versions. When using RB lenses the RZ67 operates in manual mode. Also, the RZ67 can use RB67 film backs with an adapter. The lens selections between the RB67 and RZ67 are almost identical. The RZ67 has additional lenses for perspective control for architecture and product shots. The newer RZ67 Pro IID is compatible with the Mamiya ZD digital back.
The choices are, for a mechanical camera RB67, for an electronic camera RZ67. The RZ67 is lighter because parts are made of plastic. The RB67 is a big piece of metal and is heavier. Identical cameras otherwise. When the RZ67 runs out of battery power the shutter speed defaults to 1/400th of a second, allowing the camera to function to a degree. With the RB67 you will never need to worry about batteries. Big thing, the RB67 can not use any of the RZ67 lenses or film backs. The mamiya website explains all this.
I own an RB67 with a few lenses and film backs. Very reliable camera.