I have only used the RB67 Pro S, and have never even held a 645 camera, but I just bought a Polaroid back for my RB for $15 from a pro photographer. I have seen them on ebay for about that. It is something else to consider if you might want to play with instant film. Also, if you call $10,000 - $15,000 pocket change and wish to experiment with that new type of photography, one of those backs will probably give you a better picture than the 645 back. (Although I have heard they are the same.) As one poster pointed out, that big WL viewer is great, especially if you are an old man like me. It is like watching a little TV when you are focusing. (Only backwards of course.) Ric.
P.S. I also use 35mm and wouldn't see much point in dropping that for a 645,