I am assuming this is for Amateur photography
I have used Bronica; and seen some results from mamiya; the lens are comparable quality )
The key difference between the 2 is that Bronica is a leaf shutter camera; while Mamiya is a Focal Plane shutter.
Hence mamiya lens are cheap and less prone to failure; but you dont get flash synchonisation at all speed. This means that it limits your ability for studio photographs with flash and flash fill photographing to some extent. The work around is to get a separate leaf shutter mamiya lens for portraits, which are a bit more expensive.
Bronica lens are more expensive; but all of them are leaf shutter with flash synchonisation at all speeds ( though why one would need flash synchonisation for a 50mm at all speed is beyond me)....
The other advantage of Mamiya is that because its a focal plane shutter body; you can use Hassy lens on them with an adapter available on the ebay. Which is a cheaper way to use one of the best lens in the business ( and easier to use as well, since focussing on Mamiya 645 via a TTL prism is easier than a hassy). You cant do this with Bronica, because its a leaf shutter camera.
Other than that, the differences are swings are roundabouts. I have read of some reliability issues with the super; and reportedly Mamiya 645 Pro is a better camera...
Incidentally; you might get a better and faster response if you post film-camera stuffs in the section for film photography...