I have both and wouldn't want to choose one over the other. The M's are much more intuitive and conducive to anticipatory or spontaneous shooting. I also use the Mamiya for street shooting and the larger negative is nice but I tend to use it more for urban or landscape work.
Having said that, the one thing I really don't like about the Mamiya lenses are the lack of close focussing and lens speed. If I could get down to 20" or so with the 43 and 80, I'd be chuffed. If they were 2.8's, I'd be supremely chuffed. The lenses do produce great b/w images as many will attest to. I also find handholding at slowish speeds to be doable, 1/8 or so is not uncommon.
Not sure if I would use a Mamiya 7 as a carry around camera because of it's size but ymmv. The Leica would get that vote.