I have a mamiya 6 and i L-O-V-E it. But bear in mind the 6MultiFormat (last model that included a strange 35mm size panoramic frame on 120 film) was discontinued in 1997 so the newest models will be at least 12 years old already which for an electronic camera is risky. As they get older, they will be more prone to problems and there is no supply of off-the-shelf spares to repair them - I doubt camera repairers will have a steady supply of broken bodies to strip for spares, whereas I expect your m6 would be much easier to get sorted in a crisis. Also, the Mam 6 shutter is entirely battery dependent, I'm not up on leica specs but I dont think your M6 will be.
The mamiya 6 lenses are stupendously bitingly sharp. Their resolution is reportedly comparable with 5x4 and, to my eye, this is very possibly true.
Stepping up from 35mm to 6x6 is a big step though ... if you're certain the square is for you then that is fine, but if you are used to the rectangle, why not look for a mamiya 7?
My experience of selling cameras to get to the next step has always ended in me regretting selling off my old gear. I know the economic climate is tight, but if you can, it would be much better to keep hold of your leica and save for the mamiya? then, if you find you are using the leica less, then you can sell it knowing you are happy with what you have.
The only bad thing about the camera is the meter under exposes by about a stop on mine, especially if the sky is bright (the metering is done through the viewfinder, not ttl,) but I just slow down the film speed setting or add a stop on the exposure compensation . It is rock solid and build to take knocks.