Thanks again for the replies everyone. I'm not too bothered by the electronic shutter on the RZ. All of my film cameras are electronic and I just carry a spare battery in case one dies.
Polyglot, the 110 lens is exactly why I'm looking at the RZ.
So my one remaining question is about the Polariod backs. There seems to be two of them? The one I can get in the kit I built is the Quadra72 which is a 4x5 back. Is, say, Fuji FP100c considered a 4x5 film? Just wondering if the Fuji FP films will work with the Quadra back. I know nothing about instant photography but it looks like it would be fun to try.
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.
The Nikon d300s is gone and a Mamiya RZ67 Pro II is on it's way to me. I got the camera with the waist level viewfinder, 50mm, 110mm, and 180mm lenses and two 120 backs. If UPS tracking is accurate it should be here on Monday. This is going to be a long 5 days, I can't wait to run a roll of film through it.
The Nikon was my last digital camera too so that means I'm now 100% film!
Yeah that's a total drag. The battery dies after only about 300 rolls and a 4SR44 is nearly 2cm long so it's real difficult to keep a spare in your bag without using up all the space for your film. And the battery shelf-life is only a decade or two, so you need to be careful of that spare! And the shutter accuracy is terrible, with errors of as much as 1% instead of the lovely mechanicals that can be as good as 1/4 stop when properly and regularly serviced.