The RZ is an almost fully electronic camera. The meter is electronic, of course, but so is the timing on the shutters. The shutters on RZ's will tend to drift less over time than the shutters on RB's (mechincal shutters should be CLA'd occasionally to maintain their accuracy), but if you lose battery power the RZ is all but dead. As long as that's not a problem for you, the RZ is a fine camera.
One thing to think about, though, is the repairability of the cameras in the future. The RB, being an all mechanical beast, should be easier to repair in the far future. At some point the only source for RZ electronics will be old RZ's; the RB will give you the option of having parts fabricated by a skilled worker.
The was my primary reason for buying an RB rather than an RZ. (I got the RB Pro-S, by the way. I like the safety interlocks it offers.) I intend to use the camera for as long as I possibly can, and having a camera that could be fixed based stricly on the skill of a craftsman rather than on the availability of electronic parts was important to me. Does it matter in the real world? Probably not...but I just felt better about it and I'm more willing to bounce the camera around in my backpack knowing that I can probably get it fixed long after the parts have all disappeared.
Two other thing to note: the RZ is lighter than the RB. The RB has more metal in the camera whereas the RZ uses more high-tech plastics. Personally I don't have a problem with the use of good plastics in cameras...you might love them (for the weight savings) or hate them (for the perceived weakness). Also, the RB requires two separate movements to wind the film and to cock the shutter. On the RZ you only have to wind the film; the shutter re-cocks automagically.
Good luck with your decision. They're both fantastic systems if the size isn't a problem.