My intention is to progressively build a system - either RB / RZ or 645 and use it with film initially, before getting a digital back. The camera will be used for both landscapes and portraits, the RB / RZ mainly on a tripod - I remember how heavy they are!
My .02 added to the mix...
First you need to decide on basic functionality issues: IOW do you need leaf shutter lenses or can you live with focal plane shutters; is portability a concern? Once you answer those questions, the selection of system becomes narrowed.
A short primer: Leaf lenses synch with strobes up to their maximum speeds. In the RB/RZ world, that is 1/400th sec, 1/500th for Hassy V. Focal plane shuttered bodies generally have slower flash synch speeds of 1/60th or 1/125th, but deliver much higher maximum shutter speeds, usually 1/2000th or more. As for portability, the 645 cameras are smaller and lighter by a little than 6x6 cameras; both are significantly smaller and lighter than RZ/RB. However, the RZ/RB has a rotating back which is almost essential for a rectangular capture format with a waist-level viewfinder.
Re entry costs, here Mamiya is going to win. The Pro TL was a great camera, but as has been pointed out, future adaptability is virtually non-existent. The good news is its inexpensive glass can be used normally on the AFD1, 2 or 3 body. (Exception is the Pro TL leaf-shutter lenses -- they require the mechanical linkage in the Pro TL body.) The RB and RZ lenses are excellent. With prices being the same, I would go with RZ simply because the glass can be used on the oldest RZ1 body through the newest RZ Pro II D body -- and an important final PLUS: RB lenses can be used on the RZ, but not vice-versa! There are a plethora of accessories for 645 AFD and RZ systems as well, adding to their flexibility.
Contax 645 AF, Hassy 50x and Hassy 20x all are excellent as well, though glass and accessories are generally more expensive than comparable Mamiya lenses. As an alternative plus, many third-party lenses can be adapted to focal plane bodies like the Contax 645 and Mamiya AFD via inexpensive adapters, notable here is the ability to use Hasselblad H or F glass on either as well as the ability to mount large format view lenses at the end of a bellows assembly on either.
Good luck!
