I used to have an RB-67 ProS and I traded it to my best friend for his Hasselblad 500c/m. I kind of regret that decision sometimes. I felt I was alot more creative with the RB than the Hassel. This is subjective of course. Image quality-wise, we did a comparison of the same image shot exactly the same with both cameras and enlarged them as much as a Besseler 23C could and we saw no difference whatsoever between the 2. I would highly recommend this camera. For what you're planning on doing with these cameras, I see no reason why one camera would be an advantage over the other, besides as you said the lens availablity. One more great, great thing about the RB (not sure about the RZ since I've never used one) is the bellows focusing. You can get in extremely close without a need for extension tubes. Good luck and post some results after you get the camera.
Ara