I've had both the RB and the RZ Pro II.
there are some things that prompted me to get the RZ. they include:
1. electronic leaf shutter. these are lighter than their mechanical RB brethren
2. 110 2.8 Wonderful lens, bokeh is stupendous, my #1 go-to lens for this camera
3. digital capability(digi backs). an eventual thing. in my case, since I'm a photo major, I'll have to shoot digi one day, and electronic shutters from what I've read stay "truer" over time/extended use vs mechanical shutters due to less moving parts
4. power winder. I use this almost all the time. Very handy, takes AA's, and winds the roll on/off for you at the touch of a button

(wind on push-button/wind-off auto)
5. APO lenses. I can't afford the 210 APO, but my friend has it, and for tight in headshots, or where detail/color accuracy is of the utmost importance(think transparencies in studio), this can be a very useful tool. I believe there is also a 250 and 350mm APO lenses as well.
6. prism's are MUCH lighter. I like to use a prism, since much of my rz's life is spent on a tripod. Some complain about the prism's being dim and un-inviting, I say the opposite. With a brightscreen in your camera, its totally fine.
7. NO ROTATING BACK MECHANISM!!! The mechanism is built into every RZ back, with the round ring(presumably from the mount plate(like the RB) built into the camera body itself.
8. PRO II backs, dual windows for seeing which frame # you're on. To many this isn't a big deal, to me it was. I shoot mostly in the vertical format.
9. RZ lenses. Some say that the glass is the same as the RB ones. Bollocks IMO. You get KL(best in RB lineup of glass) in EVERY lens in the RZ range. 50 ULD is superb. Zero distortion that I can see, and super sharp, not too contrasty, great modern coatings, and free floating element(ULD only!).
10. You can use RB lenses(although you can't use the scale on the side which gives you hyperfocal distance. You HAVE to use the ground glass to focus.
11. PRO II model offers 1/2 stop shutter speeds. If you shoot chromes, and want them 'just right', this can be a great tool. For digital use, it is an almost need IMO.
Downsides to RZ from what I can tell
1. Little switch on side to let you rotate the back. On the RB you can just rotate as you like, on the RZ, there's a little switch to flip. Can be annoying sometimes, most times though, its fine.
2. Batteries. You'll need a little PX28L(or equivalent) for the RZ bodies(Pro, Pro II/IID[digital-re-designed newest version]). If this goes dead, your camera does as well. I've shot more than 100 rolls of 120/220 through my RZ body since putting a new battery in there. ALWAYS keep a backup, since there isn't a battery meter in the camera

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3. Electronic bits and bobs. These mean more expensive repairs. Generally YOU can't do it yourself. Just sayin, I'm not the adventurous type to go diggin' around in my camera. I'm not built that way. I'm a photographer, not a camera technician

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4. Mechanical shutters. The RZ has a 1/400th mechanical setting(orange dot on power switch. This is for when you have no extra battery IN-STUDIO, where shutter speed doesn't really matter(with most strobes at least). The RB is the camera I'd recommend if you like to do long exposures(longer than 30s, my rz starts beeping for some reason). Also, mechanical reliability is better in cold weather from my experience. I haven't taken my RZ into a snowy, cold environment, but my RB took 15def F perfectly fine in the Eastern Sierras last winter. Mechanical reliability is a good thing sometimes

5. Price of used RZ gear vs. RB gear(backs and lenses especially). RB stuff is much cheaper to get started with than RZ equivalents.
other than that, there are probably other things I could nit-pick on, but I won't bore you. The RZ seems to be a little lighter in weight than the RB(more plastic in place of formerly metal components).
If I still had my RB, I'd probably use it more than the RZ to be honest, since most of my stuff I'm shooting right now is longer exposures(1-5 min). But oh well, whoever it was on ebay I sold it to is hopefully getting some good use out of it. I know I did

!
-Dan
EDIT: if you plan on using a digital back one day, either camera will work. However I'd go for the RB67 Pro-SD(most recent RB model) if you go for an RB67. Mamiya/Leaf/Sinar all make adapter plates for
BOTH the RB and RZ cameras. So you can adapt a back to either one.