Perhaps I am misunderstanding your question, but the M mount is Canon's mount for their M series APS-C crop cameras, which has nothing to do with Panasonic and Leica cameras. Panasonic, which was until now making micro 4/3rds cameras, has created a partnership with Leica in making a FF camera, thus they are using the L- mount that Leica already has established for their FF cameras. Lenses for FF cameras are normally wider and heavier as they need to project a wider image circle and are faster (larger apertures) than lower level consumer lenses made for the crop sensor M system. The L mount has no advantage that I know of offer better IQ. In fact, it can be argued that Canon's and Nikon's (new mirrorless) mount, will have the advantage (albeit small) when it comes to designing lenses with high IQ at a reasonable price.