As a fairly long-time Mamiya user, these are simply my opinions:
Apart from both being rangefinder-type cameras, they are extremely different tools. If your goal is agility, colour output, and less "hassle", the M9 will likely suit you better. It's of course much better in poor light also, and a much, much prettier / better-built camera.
If high resolution, or state-of-the-art Monochrome output are your goals (and for this, you should preferably be making analogue prints in a darkroom) a Leia M9, or any other 35mm-sized digital camera, will achieve nowhere near the quality obtainable from the Mamiya 7.
People (especially people who just paid $7000 for an M9) will argue this to no end, but there is no doubting an almost 5x greater film area, coupled with the superior non-retrofocus wide angle lenses.
Also, consider this: If you are used to a professional SLR, moving to a rangefinder-only body (1950s technology) coupled with a digital sensor that is demonstrably inferiour to all but the cheapest SLRs', might not be all it's cracked up to be. I would certainly keep an SLR (or modern mirrorless EVF camera, if you wish) around for the times when you need accurate framing, close-focus, and long lenses.
Regardless of the spectacular (fast, small) lenses on the Leica, by owning a Mamiya 7 you have already been spoilt by much better potential image quality in all measurable aspects, and I simply don't see what you'll gain, unless you "have to have" the beautiful Leica man-jewelry (and I must say, I would really, really like one, regardless of all the sensible reasons not to).
If I were you, and looking for some fresh inspiration, I'd go back to dakroom printing, and spend a small fraction of the Leica money on the spectacular, not-equalled-anywhere-in-the-Leica-stable 43mm lens for some ultra-wide work.
Of course, only you know what'll make you hapy in the end, Have fun either way!
