Andy, I have had great experience with both a mamiya 6 and Hasselblad 6x6 systems. The mamiya 6 MF rangefinder is the quickest, quietest, and easiest to use system I own. The 3 lenses available to it are TACK sharp and as they are symmetrical designs and not retrofocal designs to work around the SLR mirrors of a Bronica, Hasselblad, or any of the MF SLR's, so they can be smaller, and create less distortion overall. The Mamiya is really suited for travel and quick shooting scenarios, and landscape type shots. It really sucks at close up and portrait work. Overall, it is a good system, but the winder is flawed, and can present some problems that there are no fixes for from Mamiya as parts are no longer made for the winder. They just cast this camera adrift for some reason. It was probably one of their best ever cameras and they just gave up on it. Oh well. Ebay sees this system commanding very high prices... Deservedly so.
The hasselblad system I have is just a versatile though. With the same 3 focal lengths covered, standard, wide, and mild tele, you can do everything with it. The Blad is bulkier, noisy, and heavy. Lens quality is superb, yet I still feel the mamiya 6 lenses are sharper! But it's tough to see without lab equipment!
The Blad is also solid, built like a tank and just a joy to use. Everything about it suggests quality. If you stick with the later CF lenses or keep your older C lenses exercised regularly, problems should be mimimal, but can still happen. The 50mm Cfi FLE lens is just stunning as it corrects some known issues with C and CF versions. I doubt there is much between this and the mamiya 6 50mm. The CF FLE, and CFi lenses can close focus down to 19" and the CFi has the latest coatings, and floating lens elements that allow the user to choose where the front elements will sit relative to subject distance. It optimizes the quality, and elliminates edge softness on closer subjects that was prone to happen on earlier lenses.
As for the Bronica systems. I have no experience on these. I have heard that the Bronica RF system has a good following, and that the ETRSi and SQA are good systems, but also not without occasional problems.
I would stick with Hasselblad for all around versatility, and longevity, but it all depends on what you need your system to do, work in extreme low temps (mamiya RF will die due to battery freezing!), shooting style, conditions of shooting etc.