I have the Mamiya 7ii and a Hasselblad 500cm. I have the 80mm lenses for both. Both are completely different cameras (as everyone keeps saying). The eye level, the waist level aspects automatically command different pictures. Focusing is certainly easier on the Mamiya. For me, 6x7 also commands a completely different frame from 6x6, for me at least. In terms if balance, composition, I tend to use a completely different center and object-eye flow on the two formats.
I find them both very easy to walk around with. After having lugged an RZ67 around by hand, I nearly laughed from joy when I first held my Hasselblad. It's comparatively dainty and light. It's a pleasure to hold. It has the mirror up feature so slap is not an issue, however you do lose sight of your image once the mirror is raised.
I still have a special affinity for my Mamiya although I am beginning to learn which souls the Hasselblad prefers to see. Tight portraits and objects for the Hasselblad, scapes and scenes for my Mamiya with the 43mm lens...
The size of the frames makes a difference for me. I can work with a strip of 3 6x6 frames for scanning and storing and I can only work with strips of 2 6x7 frames for scanning and storing. For printing, I prefer a 6x7 negative.
If I had to choose one, I'd take my Mamiya. It also has a light meter that comes in handy.