Lots of things to consider when choosing a medium format camera.
-What size negative do you want? 6x7, 645, square format, etc.
-What type of medium format would you like to shoot? Different sizes and shapes tailored to different shooting styles. (SLR, TLR, Rangefinder, etc.) Some are more portable, others weigh a ton.
- How many frames do you want per roll? You get 15 frames from a 120 roll if you use a 645, but that same 120 roll will only get you 10 frames if you use a 6x7.
- How much automation are you looking for in terms of metering and aperture control?
- How much are you willing to spend on accessories like lenses, prisms, film backs, etc.?
I have used medium format for years, mostly Mamiya (645 and RB67). I prefer my 645 because of its smaller size and weight compared to the RB67. I also found that the 645 lenses have better coating and produce richer colors than RB lenses. The 645 handles much like a regular SLR film camera, where the RB is a more studio-oriented camera (shutter speeds are on the lens, metal film plate has to be removed prior to taking a shot). I have the CDS metered prism on my RB67; camera with lens, 120 back and CDS prism weighs 7 pounds. The RB67 has a rotating back which means you can shoot either portrait or landscape orientation. A very cool feature when your RB is on a tripod.
Lenses and accessories for Mamiya 645 and RB67 are not very expensive. You will find that if you go with Hasselblad or Contax you will be spending way much more, especially on lenses.
For many years I produced large prints with both systems (645 and 6x7). Depending on the film you select (Kodak Porta line of films was my favorite), you will be able to get outstanding negatives capable of huge enlargements (or equally huge scans).
If you want to take a dip into the medium format world I would suggest looking into a Mamiya 645E with 80mm lens, 120 film insert and rapid film advance handle. The limitations would be that you will only be able to change film inserts (either 120 or 220) but not be able to change film backs (you wouldn't be able to add a Polaroid back), the prism is part of the body (you won't have the choice of mounting a waist level finder or magnifying accessories), and the camera is battery dependent.
The advantages of a Mamiya 645E system are that you would have access to Mamiya's fantastic lenses, you would be shooting medium format, the camera has manual mode and aperture priority mode and the size/weight is manageable.
p.s. forgot to add: The cool thing about having several film backs (instead of just film inserts) is that you can pre-load several backs with film making it more convenient to shoot more rolls quicker. This is especially convenient if you are shooting an event (like a wedding) and know in advance that you will be using several rolls. Also, most 120 rolls need to be kept refrigerated.
