I'd go for the RZ. I have one and it's superb. I just have the Pro(I) version, but it still takes the latest AE-II finder fine and all functions work (half-stop speeds, AE/AEL, etc.). The I doesn't lack much - fine focusing knob, half-stop speeds on the dial - that's about it. Id' look for an RZ Pro (I) - cheaper and basically as good as the ProII for what you want.
The 110 is a superb lens. The best in the line I think. 50ULD, 350 and 500 apos are also sweet. RZ is lighter and smaller than the RB, so that's a plus.
Note the 6x8 back is only 74mm wide, so only 4mm wider than the normal 6x7 back on the RZ (70mm).
Can't really go wrong with any of them, but to me I think the RZ betters the RB in many significant ways.