I was recently in the same position, wanting to try medium format, looking at the Mamiya 645 and the Hasselblad 500CM, and not being sure which I preferred. Having come in to a small amount of money, I did a wicked thing (if you believe The Boss) and bought both, a Mamiya 645 Pro with 55/80/150, and a 500CM with 80/150. So far, I'm only in play/familiarisation mode, so I'm only offering impressions rather than advice, of course. I found the transition from 35mm SLR to Mamiya 645 very easy. Instant return mirror, mirror lock-up, AE finder (very affordable in Mamiya). Changing backs on the 645 is a breeze, and the Mamiya has a place to stick the darkslide when the back is ready for use, whereas the Hassie doesn't. The Hassie is requiring more 'dry use' familiarisation, since you have to keep the shutter cocked state of the lens in synchrony with the wind-on state of the back. The mirror on the 500CM is not instant return, and instead of mirror lockup, has a pre-release button, which moves the mirror out of the way ready for separate firing of the shutter. However, it's not the same as mirror lockup, because you can't bring the mirror down to re-focus or re-frame without firing the shutter, as you can with the Mamiya. The Mamiya backs have individual ISO settings that communicate with the meter prism, so swapping backs is a breeze as the ISO is automatically set. The Hasselblad lenses use that EV system, where you set an EV on the lens and the speed and aperture are coupled, and you need to press on a release in order to decouple the shutter speed settings from the aperture settings. As it happens, I'm used to that because my little Yashica rangefinders use that system, and I rather like it, but it's just something to be aware of.

If you prefer, say for flash synch reasons, leaf shutter lenses, be aware that there are a few Mamiya 645 lenses with leaf shutters too. Shift lenses on the Mamiya won't break the bank or strain marital contentment states (OK, in my own situation I may now be stretching that latter claim).

My entire Mamiya 645 Pro with WLF, AE Finder, power winder, three lenses, electronic release, three backs and inserts, 220 insert, cost significantly less than the 500CM/80 with one back. Beyond the glass (I discovered Zeiss about a decade ago and now I understand), I'm finding it more difficult than I thought to choose, because each has strengths in different areas. And whilst my save-in-a-fire choice would be the Zeiss, the Mamiya lenses are no slouches.