I can only offer my observations with the equipment I have (M8 and Canon 20D/5D-I), and like Jaap said, in the end it comes down to a matter of personal taste, eg. I have never found the prints from the Canon files any less sharp than prints from the M8 when both are processed with workflows geared specifically for each.
I have used the 400/6.8 (what I call the Trombone Telyt) on the M8 (Viso-III masked down for the crop) which gives an effective 530mm. And I've used that lens on the 20D (effective 640mm) and 5D-I. The IQ on the M8 and 20D are both excellent at lower ISOs, but given the lens's slow speed and somewhat ungainly ergonomics I like to use as high a shutter speed as I can, therefore the 20D's superior lack of noise at ISO 800 and 1600 gives it an edge (plus it's a lot less cumbersome vs the Visoflex setup). On the 5D, the Telyt's pronounced field curvature is more evident due to the larger image circle, and I would expect that to hold true with the M9 for the same reason. Still, the performance and utility of that lens is so good that I sold my Canon EF 400/5.6L. PS I use a 72-67 stepdown on the front lens-retaining ring threads, and a 67mm IR filter for the M8, but an E49 with the male threads ground off (or the glass transposed into a Series-7 mount) would work in the rear slot. Two added advantages of using the lens on a Canon are that I can use AE (never seemed to get it to work well on the M8 just after the mirror trips no matter how carefully I adjusted the trip arm of the Visoflex), and also the very good EF-1.4x Canon teleconverter, which I own, rather than having to buy an expensive Leica 1.4x-APO-R.
My only other Visoflex-capable lens is the 135mm Tele Elmar head on a Universal Short Focus Mount. I have used that also on the M8+Viso-III (180mm effective), and both the 20D (216mm effective) and 5D-I. There isn't much that can be said about the 135 T-E other that it is sharp and contrasty and near-APO quality, even wide open. I have only one other prime 135mm lens to compare to that I can mount on the Canons, an old Nikkor f/3.5, and as expected, the Leica lens is far better in all respects. I have the Canon 28-135/IS and 70-300/IS but although they are great performers they are not L lenses, let alone not primes. The hassle of un-doing the lens head and the baffle and switching to the short mount is enough of a drag that I rarely do it, so for the most part the T-E is used directly on the M8. I have an old Canon rangefinder 13.5cm shoe finder whose eye relief is such that because of my glasses, limits me fortuitously to an almost exact 180mm view.