a Hasselblad 500 with an 80/2.8 and a loaded film back weighs 2.5 pounds less than a RB67 with a 90 and a loaded film back.
a Hasselblad 500 gives you an extra 2 frames per roll, making reloads less frequent, which is a plus when you're walking around with a skeleton load (no camera bag with extra backs, just a couple extra rolls of film in a jacket pocket)
a Hasselblad 500 is physically quite a lot smaller.
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a Mamiya RB gives you a bigger negative, both in actual size, and in cropped proportions, if you prefer to print rectangles (I print square negatives as square pictures, so not an issue for me).
a Mamiya RB has less expensive lenses on the used market, giving you more FL options...65 + 90/127 + 180 can all be had for the price of an 80 + 150 in Hasselblad land.
a Mamiya RB can focus to dramatically shorter distances due to the bellows, if you do a lot of close-up work, consider this.
a Mamiya RB with a standard film back, doesn't advance the film and cock the shutter in one stroke, whereas the Hasselblads do.
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I've owned both systems...sold off the RB67s, but still occasionally use my dad's RZ67 ProIIs (much more comparable to a Hasselblad in terms of actual ease of use, in my opinion)
I prefer Hasselblads to Mamiyas on all other fronts, regarding the overall gestalt, feel, and operational aspects.