I use both an RB67 and Mamiya 645 Pro.
The advantages of the RB67 are:
1) larger negatives/transparencies, with all the inherent resolution, detail and "tonality" that results;
2) rotating back that makes use with a waistlevel finder or chimney finder simple and easy;
3) leaf shutters on all the lenses;
4) lenses that have contrast and resolution characteristics that I particularly like to print from;
5) common filter size (77mm) across all my lenses;
6) 6 x 4.5 backs that give me the option to take and project slides in my projector; and
6) a hand grip with built in shutter release that works for me.
The advantages of the 645 pro:
a) negatives/transparencies that are large enough to assure excellent resolution, detail and "tonality";
b) when used with a grip and prism finder, reasonably easy to use in either portrait or landscape orientation;
c) reasonably small and light, when used with a prism finder. Lenses and accessories also reasonably small and light;
d) good metering results with a metering finder;
e) inexpensive and plentiful used film backs and inserts;
f) slides that will work with my medium format projector!;
g) a roll of 6 x 4.5 negatives or slides will fit in a single Printfile negative/transparency sheet, without any orphan leftovers; and
h) a hand grip with built in shutter release and hot shoe that works for me.