I have both cameras...an M2 and a Mamiya 7.
For me, the Leica feels superior in every way. Weight, heft, build...while the Mamiya is mostly plastic. A beautiful piece of plastic it is though, and gorgeous negatives.
But then there's that Leica shutter...perfect! Like a kiss!
The Mamiya shutter is so unsatisfying, like a toy really...enough for me to want to sell my Mamiya 7.