I had always thought that RB glass would be a lot cheaper than the newer RZ glass. After really looking into it for the first time I was quite surprised to find that it wasn't really the case. I guess something just feels wrong to me to pay more (or even the same price) for inferior glass. I figured that if I can swing the extra money for an additional wide lens, I would be making a more long-term decision.
The biggest investment in camera systems is usually the glass, and I feel that buying RZ glass will be a better use of my money. The camera solves a couple of the issues I have with the RB (inconsistent exposure among lenses, two-motion cocking, etc) and I think I would be money ahead going into it now rather than later, especially if this job will more than pay for the gear.
I realize a lot of people disagree and the RB67 seems to be generally more liked on this forum than the RZ67, but I gained a lot of insight from the comments. Several good arguments were made against the RZ67 system and it helped me make sure I was thinking about everything. The downsides of the RZ that most presented would not be downsides for me. I do definitely agree that it would be a bad choice if I only had $300 to spend and chose to spend it on a new camera that still did not have the lens I needed.
I think perhaps I incorrectly framed the question to suggest that I could spend either on a 50mm lens or an RZ67 with a 90mm lens. Since I can do both, I think I'll be happy with the choice.