Get the RZ, if only because it means you can use the 110/2.8 - the smallest, lightest, fastest lens in the system. Don't bother with prism initially unless you do a lot of headshots and don't want pictures of nostrils, otherwise use the waist-level finder. If you get a killer deal (i.e. free) on an RB lens, you can still use it on the RZ body.
As for batteries, quit whingeing. A 4SR44 lasts for many hundred rolls and you can shoot probably 50-100 rolls while the "replace battery" warning is showing; the cheapest 4LR44 is good for probably 200 rolls and you can keep a spare in the corner of your bag, it's the size of two peas. I would trust the reliability of a quartz electronic timer far beyond any clockwork shutter, especially when it comes to wear and timing accuracy. An electronic shutter will never drift or be inaccurate. Electronic metering can be flakier due to bad contacts in the backs and the prism, but you don't get that at all with RB.
The only good argument I can see for the RB is the 70mm back but I think you can get an adapter to permit its use on an RZ anyway. And 70mm film and development gear is hard to come by now.
There is no 70mm-argument for the RB. with rz-g-adapter it can be used. 70mm films are available and one can develop them ourselves. with jobo 2517-reel in testdrum(10x40cm printtank-only 500ml)or in 2500-tanks. I dont know yet how cut-down perforated 70mm will work in the lab. it will show remaining perforationswhich could damage the machine. xkaes on ebay has slitters of any size-combinations or can produce them.