I've got both the M-Rokkor 40 for the CLE (the one with the easier-to-find 40.5 mm filters) and a Version IV 50 Summicron. On the M3, I prefer the 50, as that's a match made in heaven.
But, overall, my most-used combination is the M6 with the M-Rokkor. The image quality is, in my opinion, every bit as good as the Leica 50. The 40 field of view is just about perfect for me--it's what I'd call my "normal" angle of view--and the lens is much smaller and more compact than the 50, while still being very well-constructed. You can easily slip an M camera with the M-Rokkor into a large coat pocket, which is more difficult to do with the longer 50. Finally, the 40 is a very good match for the M6's 35 frame lines, at distances over a few metres; it's trivial to remember to be a little more generous with framing when shooting at close distances.
The one drawback is that the 40 Summicron-C/M-Rokkors bring up the 50 frame lines automatically on M cameras, rather than the 35s. The lenses can easily be modified, with a bit of filing, to bring up the 35 frame lines. But if you don't wish to make an irreversible change to your lens, I found out online about a kludge that works quite well for me: if you take a plastic film canister cap (the Ilford ones work particularly well, as they are about the perfect thickness and are black) and cut it into pie-shaped wedges, you can use one of these, with the rounded end braced against the lens mount, to hold the frame preview selector lever in the 35 position. Works great, requires no surgery to the lens, and is effectively free!