I have both. I like them both.
Main reasons for the RZ: the 50 uld and the 110/2.8. A little bit lighter. Single stroke lens cocking and film advance.
Main reasons for the RB: 6x8 back, operation without battery, and much more favourable prices on new lenses and peripherals.
Note that the newer (KL) RB lenses are as good in all respects as the latest RZ lenses, but the lack of a fast normal (110/2.8) in the RB lineup is a big deal for some people.