To me, the thing that marks the medium format experience is the feeling of expansiveness, closeness and detail that is visible through the finder and the elegance of the shape relative to the angular, distant and cramped (my view) 35mm view. I get these things with 6x6 and 67. Personally I don't get it with 645 which I guess is another way of saying that it all looks too similar to 35mm for me. That said there is certainly enough film size to produce great large prints.
If you do go 645 I'd suggest you take a hard look at the Pentax 645n. It handles very much like a 35mm; has great auto exposure if you want it and the lenses are very good indeed- even the zooms. Definitely a generation ahead of the Bronica and Mamiya models.