Maybe I missed this--BUT what does the RZ get you that the RB doesn't other than the ability to put a battery in the camera. I have shot a RB for 10 years and the camera body has never failed to deliver. Good old mechanical simplicity. Some have stated that the RZ lenses are better, which on paper may be true but there has never been a softness problem with the RB lenses I use. (50mmC, 90mmC, & 250mmC)
I say just get a 50mmC and keep using the RB it is a tried and true workhorse of a camera. Bang for the buck.
RZ gets you the 110/2.8 and the 50 ULD - in my opinion both valid reasons to get one! It's also a little lighter, doesn't require regular shutter CLAs and you can use the AE prism if that's your thing.