Having *owned* an RB67 and RZ67 kits in the recent past, and now having chosen the Hasselblad, here's some pro's and cons from my POV.
Positives(IMO) *for Hasselblad*
Lighter in weight than the RB/RZ system
Smaller footprint(size) than the RB/RZ
No batteries(unless you have a motorized body, such as a 500ELX, 553elx, etc...). The RB67 is also fully mechanical
Lenses are smaller(but not necessarily lighter)
Lenses are slightly lower in contrast(IMO) than Mamiya glass. I shoot chromes, so this helps tame contrast a bit.
Rentals are still available for lenses/bodies, etc.. in many major cities worldwide(if needed)
Negatives(IMO) *For Hasselblad*
Much more expensive(usually 1.5-2x more). Almost everything is this way pricing-wise.
Smaller negative. Having a 6x7cm negative/chrome allows one the "ideal" format, and you can always crop it square in post/printing/scanning.
Positives(IMO) *For Mamiya*
Relatively inexpensive to build a nice system
Cheaper accessories (cost-wise)
Rentals are still available for lenses/bodies, etc.. in many major cities worldwide(if needed) (this is relating to the RZ lineup, not many places(if any) RENT RB67 equipment)
Higher contrast lenses(good if you shoot lots of b/w and like more "bite" straight out of the film. This is from my experience.
6x7 is the "ideal" format
Interchangeable lenses(RB lenses work on both RB and RZ lenses, RZ only on RZ bodies)
Bellows focusing(good for close-up w/o having to use supplementary extension rings most of the time)
Negatives(IMO) *For Mamiya*
-Heavier
-Bulkier
-Takes batteries(RZ67 line of bodies). Shooting in the field, and not having a fresh battery can be a real buzzkill for a landscaper(like myself). I'm speaking from experience here
-Bellows focusing(good for close-up w/o having to use supplementary extension rings most of the time). Have you any experience with a view camera? Bellows extension means you'll have to do exposure adjustment if focusing closer than infinity. The scales on the side aren't very clear IMO.
-10 shots/roll for 120 vs 12 for the Hasselblad(6x6). But you get the "ideal" format
hope this might clear some things up? I'm sure there's other things, but pro's and con's are subjective, and vary from user to user. These are just my major ones for each system.
-Dan