I have a M6 and find it to be once in a life time camera. I have a 28 and a 90. this is my street camera. BUT i just got a Pentax 67II and even though it is a little heavier than the mamiya 7 which from my investigation is suppose to be a better camera I got this one. they both have there areas of use but getting rid of one for another isn't I think a replacement. I shoot from 35mm to 8x20 and everything I shoot with has it's own purpose. If I go down to the shrimp boats to photograph with my LF cameras and Ulf cameras you will get a certain % of shots. but a 6x7 will get a whole lot more and I'm not sure that it can't do as good a job.
So that is my thoughts I hope it helps.