The Mamiya certainly has advantages for low light. I don't remember if it has been mentioned, but focus will be quicker with the rangefinder. I was going to say "easier", but that is not so true with the waist level finder (w/magnifier) on the Hasselblad (but the lens focus is still one of the slowest around, at least on the old CM lenses-not sure about later ones). If you want eye-level viewing, I also think the Mamiya wins out (to me the Hassleblad is at its best with a waist level finder). And I agree they were a favorite of many wedding photographers (probably more the 6 than the 7 though for that use).
Still, I never quite warmed up to the look of the Mamiya lenses (I owned the 6, but also got to shoot with the 7); they are some of the finest lenses ever made, but I still like the look of the Zeiss lenses better. I also like waist level finders, so guess I'd be in the Hassleblad camp. It really is a wonderful and sold camera with great optics. And lately they seem to go begging at below rock bottom prices. I don't need any more cameras, so do my best to ignore the bargains I hear about.
BTW, I'll second the suggestion of a Rolleiflex w/2.8. I think they beat either of the others for low light, and still have that great Zeiss (or Schneider) look. The late models also take a wonderful prism; that is my favorite setup.