The most important practical differences to the RZ are that shutter/mirror and film transport are two different levers on the RB (unless you use the motorized film holder on the Pro-S or later models), and that the RZ has electronic shutter control from the body, while you have to set the time on the RB lenses themselves.
Besides that, RB lenses are mountable on the RZ (at the price of a considerable loss in comfort and with one or two exceptions for mechanical reasons), while RZ lenses cannot be used on the RB at all (the RB lacking of any way to control the RZ system shutters).
The RZ is more comfortable, feels more familiar to 35mm SLR users, and has a few newer and better lenses - on the other hand, RBs and their accessories are massively widespread, more rugged and quite considerably more affordable.