I would recommend the RZ over the RB. Sure you may find a used RB a little cheaper. The fact that it doesn't use batteries is nice.
While using RZ lenses with an RZ when you advance the film it also cocks the shutter. Yes, I realize cocking the shutter is no big deal.
OK, here's a big one! With the RZ you have the choice of using both RZ lenses and RB lenses. Of course you have to cock the shutters on the RB lenses.
The 110mm standard lens for the RZ is fantastic plus Mamiya has a floating element 50mm wide angle for the RZ if you like wide angles.
Just something to think about!