While (almost) all MF cameras use 120/220 film, they produce images of different size and ratio on the negative. 6x4.5 cameras are usually horizontal (landscape) in format and you will get 16 images per 120 roll, 6x6 are square and yield 12 images per roll, 6x7 will either be horizontal or have a rotating back and yield 10 images per roll. The larger the negative, the greater the resolution of the final image- and, in the case of SLRs, the larger the camera that takes it. Although all of these formats are a big step-up from 35mm. Some people look down at anything less than 6x7, others (such as my self) love composing with the square image of 6x6. 6x4.5 SLR's tend to resemble 35mm cameras in handling the most. I don't own either, but limited experience and reading has indicated to me that both the Bronica ETRS and Mamiya 645 systems are highly capable, versatile and light. Others with more experience using those systems could speak to the differences better than I, although I've thought about getting a Mamiya 645 at some point just to use the 80mm f1.9 lens, which is the fastest regularly available lens for any MF SLR.
Originally Posted by James-EG
If you have a great price on a Bronica ETRSi, I'd say go for it. It's usually possible to resell equipment for little, if any, loss if you decide another system meets your needs better.