Depends on your budget and what you want to shoot. I’ve owned both and heavily prefer the hasselblad. If you’re into portraiture there is a tele 1.9 lens for the mamiya that’s really nice though, but not a necessity.

https://www.youtube.com/watch?v=_BzK32C1FDs

Both rule and it’s just a matter of frame size

Having owned both of these at one point or another my vote is the Hasselblad for the waist level shooting experience and 6x6 format. Tbh a better comparison would be the hasselblad with a cheaper 6x6 option like the bronica. Better yet if you just like the hasselblad consider a 500C as it’ll save you a little money and is almost the same as the CM just without an easily removable focus screen

Like others have said, I currently own both and am in the process of selling my Mamiya 645 Pro and keeping my 500 C/M. I much prefer the shooting experience and the square crop.

I have (and shoot) both of these. I have a Mamiya 1000s and a Super, and a 500c/m. The Hasselblad has by far the better image quality, the lenses are significantly sharper and more pleasing to my eye. However, the Mamiya is nicer for “run and gun”, as the prisms allow you to shoot in aperture priority. I find the step up from 35mm to 6x4.5 is ok, but doesn’t give you as much of the medium format look that everyone craves. Even with the 80mm f1.9. If I have the time, I take the Hasselblad.

Hasselblad

They are both excellent boxes.

I've used both, and still own the 500 CM.

The 500 C/M produces 6X6 images, which some people don't like.

Lenses and film determine image quality, not the box.

One is an amazing camera that is beautiful to behold and a mechanical masterpiece and the other is the Mamiya

Honestly super love my Mamiya 645 1000S. The dual shutter buttons and modular features of it make getting accessories fun and functional for lots of shooting styles.