I may be a little late to this thread but I think it's important to decide in this order (imo):

1. Type of camera you feel most comfortable with since photography is as much about you as it is about the subject. Do you want an SLR-esque camera? A "classic" medium format camera with a waist level finder? TLR?

2. What film format you "see" best in. As people have mentioned this can be 6x4.5, 6x6, 6x7, 6x9, etc. Personally I prefer 6x7 because it's much larger than the 6x4.5 negatives and is an "ideal crop" format which makes it much easier to find frames.

3. Budget. Self explanatory

FWIW I have a Mamiya RZ67 with 110 2.8 and 50 4.5 lenses. It is a GREAT camera and really is not as heavy as people say. I've been using it nonstop for almost a year and weight was never a concern with me. It is 95% hand held except for night shots (where a tripod would be almost mandatory anyways). It's no featherweight, but I feel people play up its weight too much.

The camera is extremely modular which is very nice. If you break a back, body, lens, etc you can hop online and find them relatively easy. This is one reason why I did not go with the Bronica GS-1, apparently parts are harder to find for it. I almost went with a Bronica SQ-Ai but decided I wanted the 6x7 format instead.

It's great to hear you're getting into medium format photography hopefully this reply wasn't too tedious to read!