I have both, and I'd take the Mamiya 7 for hand held low shutter speed shots, especially outdoors. The problem with the Hasselblad isn't the shutter, but the mirror. I estimate the Mamiya gives me 2 stops better results than the Hasselblad. For 8x10 output, with an 80mm lens on the Hasselblad I try to keep the shutter speed above 1/60th (ideally 1/125th), but with the Mamiya 1/15th to 1/30th is as slow as I would go. Also keep in mind that you are going to enlarge (and crop) the Hasselblad image more than the Mamiya 7 image for an 8x10.

To me you should really be looking at how the lens draws it's image. I think the Hasselblad is much nicer when used close to wide open. But the Mamiya 7 is sharper, more clinical looking. I really like both systems.