I own a Hasselblad system and a Mamiya RB67SD (effectively like a RZ but no batteries). I'll start with the Hasselblad.
Hassy----
I have a Hasselblad 501CM. I have owned a Hasselblad system for 5 years now. Started with a 501CM, then replaced it with a 500ELX, then went back to a 501CM after having to repair the ELX 3 times for the same fault. Since we're talking repairs I also had to fix one A12 back for a broken frame counter (bought it like that so fair enough) and my current one probably needs to go sometime for service as I get the odd spacing issue. I have also owned at one time or another the 50CT, 50CF, 60CB, 80CB, 150CT, 150CF, 160CB and 250CT. I have the 80 now only and the 160 is on sale. Anyway...wonderful system. I use it exclusively with a PM90 prism. The tactile feeling is superb. The scans and prints I get back are just amazing. That Zeiss glass is really amazing.
HOwever...things I had to look for. For starters, unless you get a body with a Acute Matte D screen (latest type) you can't focus sh!t (well, I at least). Unless you want a split screen but I don't like these, I prefer a plain screen so I can see all around. So that's expensive if you can find one and is not included with your camera. But it is the brightest screen I've ever seen (and I have a Maxwell one so I'll come to that later). Also, servicing one of these is more expensive than most other systems.
So, overall, great system, great picture quality, the ergonomics are mostly ok though I hate that shutter button that gets tripped when it catches on my belt as I walk around (camera over shoulder) and I really find the 1/60 min shutter speed (handheld, no time for tripods) limiting. And it is expensive.
Mamiya RB----
I bought a like new (I actually think it was unboxed old stock, it was unmarked) RB67 SD last year. I now have a couple of backs, prism and have owned the following lenses, 65KL, 90KL (new), 127KL (also new!), 180KL and 250KL. These are all the latest optics and I believe are the same as the RZ optics. Ok, the camera is massive. I mean friggin huge. It makes the 501CM look like a toy, especially when I have the grip and prism on the Mamiya. However, a RB with WLF is not that much bigger in volume compared to a 500ELX.
Handling: it is heavy. I find it tricky to use without a grip. With the grip I can carry it around all day. The strap is crap though, it won't rotate and whichever of the four angles you set it to (once it is set that's it, you have to remove it to adjust it) it will be wrong, either for shooting or for carrying. So grip it is for me. Alternatively I will leave the grip home and put the camera in the bag, take it out, shoot, put it back in again. Also, without the grip focusing close makes it very front heavy and a bit unstable due to the bellows. The rotating back is just really cool. And I can shoot down to 1/15 and even 1/8. That's two stops over the Hasselblad.
Finder: When I first opened the finder I thought WOW! Hasselblad schmasselblad. That screen is like a TV. it is dark though so I eventually got myself a Bill Maxwell. Now Bill is a nice guy and that screen for the RB is very good but it is not $400 good. He tried to sell me a screen for my Hasselblad. Claimed it would be 10x brighter than my Acute Matte D just like the RB is. Well...the RB screen is not brighter. Period. It is almost as bright but not brighter. And reflects more light when looking down (probably because it is not brighter!). But the size of the screen makes it p1ss easy to focus. It is fantastic! I can reliably focus anywhere on the screen. So I would budget for a screen from Bill, I hear he makes a cheaper version (near half price) just as good nowadays...
Speed of operation: Probably as slow as the hassy. The focusing takes for ever as it is bellows. However I find the wind-on quicker (even the two step process the RB has, body+back, the RZ is single crank0. Push and done. The Hassy is twist twist twist done. I can even do the RB without taking my eye off the finder (when I have the grip of course).
Image quality: whoever says the RB/RZ optics (latest type, don't know about the older ones) are inferior to hasselblad they clearly haven't used one and are just brand snobs. The results are AMAZING. The clarity is there. The contrast. The bokeh at close-up is cream and butter. But it is a different look to the Hasselblad. The Mamiya has a more modern look, the Hasselblad is more traditional. One is not better than the other, they have a different look. Besides, one is square, the other is not!
Prices: the total cost for my **new** RB67SD, two backs, prism, 65KL, 90KL, 127KL, 180KL and 250KL is the price I paid for the 501CM+80+A12 kit. Enough said. I paid £80 for a like new 180KL. £60 for a mint back. In hasselblad-land £60 buys you a scratched lens filter. At least in the UK.
Which to have: I thought about selling the hasselblad. It collects mostly dust now that I have the RB67. I actually find the RB a nicer handling camera. It is heavy but it is actually smoother to focus (slower but lighter focusing) and the wind-on is simpler. The shutter release is soft and damped. The screen is massive. It costs nothing to buy more equipment. Servicing is cheaper. I can do close-ups without tubes or filters like you need in the hassy. I can shoot in lower light. I can shove it in a ThinkTank Retrospective and walk around for hours. My Hasselblad is a £1300-1400 (with all the accessories) collecting dust (haven't used it in 4 months). But after a lot of thought and as I was seriously tempted to sell the Hasselblad I decided to keep both, mainly on the advice of the missus (!). Different systems for different things!
It is like selling your children these things and while you know the Mamiya is the one that will do you proud every time the Hasselblad is the slightly spoilt, slightly annoying one you always forgive as it's so pretty...