Um, if you're contemplating 645 why not go the whole hog and use 35 mm?
Sound silly, but in his book Field Photography A. A. Blaker makes the point that when going up in format increasing the size by less than twice in both directions (horizontal, vertical) makes little sense. The logic works for going down in format too, 24x36 is more than half as big as 645 so the loss in quality in the final prints probably won't kill you. And a light little 35 mm SLR has the size and weight advantages over a Mamiya 645 has over an RB.
Applying Blaker's logic to the 645 vs RB 67 decision, 645 is half frame 6x9, i.e., more than half as big as 6x7. So there 645 may be the better choice for you.