I'm probably biased towards the RZ, as that is my favorite. I have never experienced battery drain issues with my RZ's. A moot point that some mention as a deterrent to acquiring a RZ, ridiculously favoring the RB on that point alone. There are times I prefer to use auto exposure; not available with the RB. Then, as mentioned, the RZ allows use of the 110mm, the 50mm ULD, the 210 APO, and the tilt-shift adapter. All with auto exposure if needed.
Big pluses in my book. A small, separate advantage with the RZ, is the single cocking lever, small issue, but still an advantage over the RB.
On another foot, one can not always say that a RZ will be newer than a RB. RB Pro SD's were simultaneously produced along side RZ's for many years.
Also, some more options to add to your decision process, there is a seller in Japan with a lot of new-old-stock NOS on the big auction site now.
Check out some of his RB and RZ gear. His auction site handle is, sanpoucamera