I had an RZ and an RB set. The RZ has that "one-cock" advance, without having to do the two stroke advance (body, then back) on the RB. The RB has the advantage of not having to flip a lever before rotating the back. The RZ feels cheaper, compared to the RB's metal body. The RZ takes a battery, the RB doesn't. The RZ takes a motor winder for the body, the RB has to have backs that auto-wind. The RZ can have on-board metering, via the prism, the RB cannot.
The RZ has a "you left the darkslide in, you idiot" indicator. The RB does not. The RZ takes the SCA395 adapter, so you can get a "flash-ready" lamp, the RB does not.
There is no battery in the RB, it'll keep shooting. In the RZ, if your battery dies, you shoot at 1/400 ONLY.
In the cold weather, the RB is more reliable. My RZ had a ton of problems (even with the external battery pack). You set the RZ's shutter speed on the body, you set the RB's on the lens.
Overall, I'd rather have an RB than an RZ. Sure, you can set the RZ to RBL mode and use an RB lens, but the whole battery dependency and plasticy feel made me hate it. That's why I went with Hassy. Hell, with the RZ & RB's prices so close together, you might as well get an RZ though and get an RB for a backup body (you can share the lenses as long as you get RB lenses!
Of course, the RZ lenses have a bit more contrast than the RB lenses of the same generation. But the Zeiss lenses blow that away. I thought it was just a tale until I shot the two side by side in studio on NPC from the same batch and printed it on Supra paper with the contrast as high as the paper could handle.