I agree -- if you already have a lens, by all means use it, but if you are going to seek out a lens and pay good money for it to use on an M camera, getting a rangefinder specific lens is a much better idea -- optically they tend to be better in most cases, they are smaller, lighter and have added function.
As for the summicron vs. summilux, it is a very long-standing and personal debate. I was a vehement supporter of the summilux for a long time, but I decided to try the summicron. I did a blind test of the images at a few different apertures. I took two images of the same scene with both lenses, labeled them, and set them aside for a few days until I forgot which was which. I could barely tell them apart, outside 1.4 to 2.5. When I looked closer the summicron had a similar macroscopic look and feel to the summilux (they are both Leica 75mm lenses after all), but on closer inspection the summicron files were sharper, perfectly consistent in their focus point and had no noticeable chromatic aberration or bokeh fringing. The summilux was softer, had lots of bokeh fringing wide open, and the focus point jumped around a bit.
Don't get me wrong, it is still a great lens with beautiful character, but if you don't already have one, I would strongly recommend the summicron for digital use. For film, the summilux is just as good...any difference in performance is usually masked by the film grain, and the focus shifting is not really pronounced due to the curvature of the film.