I have used them both. I own an RZ because it came onto the APUG classifieds as a kit, and was within my budget. Before that, I had often used borrowed and checked-out RBs (Pro S models).
I like the RZ. I prefer the fact that the RB does not need batteries, and that I can shoot 6x8 with it, which is a FAR preferable aspect ratio for what I shoot most of the time. I may be able to shoot 6x8 on an RZ, but have not figured out how yet.
I would prefer to have an RZ II, for the intermediate shutter speeds. I am not complaining about any other model, however this is a feature I would take advantage of all the time.
I like that the RZ can easily take RZ or RB lenses.
I prefer the mirror lockup ergonomics on the RZ. Insert a cable release, hit the button that is normally the shutter release to flip up the mirror. Hit the cable release to shoot. Advance lever brings the mirror back down. Very quick and easy for me to execute (and remember).
I would definitely prefer an RB if I did a lot of very long exposures (minutes or hours versus seconds).
The RZ looks and feels cheaper, but I think after using it that it is just cosmetics that make it seem this way.
RZs and RBs are not as far away in price as they might seem. I never even considered an RZ until I saw that they had come down in price so darned much. I got my RZ kit with a normal lens, two backs, three inserts, Polaroid back, and a messed-up 250mm RB lens that only shoots wide open for what I had expected to invest in a slightly less full RB kit.
I would not call either one of these cameras heavy or beasts. Heavier than a 35mm? Of course. It is a 6x7. But not heavy period.
The most important features to me on each model would be the intermediate shutter speeds of the RZ II, and the 6x8 capability of the RB. Other than that, there would not be that much practical difference for me. I would grab whichever camera came my way first...which is what I did.
One thing that I despise about both cameras is the focusing scale and depth of field indicators. Same gripe I have with the C series TLRs. It is near impossible to quickly and perfectly set focus to an exact distance, to zone focus, to hyperfocal focus, to tell what distance you are focused on, to preset focus, etc., etc. Every lens is crammed onto a sliding focusing scale (so the longer the lens, the easier it is to use). It is the only part of using the camera that I would call "terrible". I would rather use a scale for each lens that slips on over one of the focusing knobs to give you a Rollei-esque scale/DOF indication. This really just comes down to bellows versus helical focusing. What you gain with a bellows, you lose in this aspect of shooting.