The RB67 lenses have in built leaf shutters whereas the 645 has a focal plane shutter. This may be of some consideration if you intend to use flash a lot as the RB67 will synchronise at all speeds whereas the 645 will have a maximum sync. speed of 1/60 (I think).
There are leaf shutter lenses for the 645 but they are not standard. If you want high flash sync speeds and the 645 format then the Bronica ETRS system might also be worth considering as its lenses have built in leaf shutters.