Conclusion
For a professional photographer who needs a system that can be used anywhere, at any time, to do almost anything, clearly the choice is the Canon. The entire concept of this camera, indeed, professional DSLRs in general, is to allow the highest degree of flexibility and versatility, in essence, to let the pro buy into one system and never have to work outside of it. The Leica is simply too limited in its set of features and flexibility of design. Likewise, a committed enthusiast is going to want the Canon. In this category, it’s likely you’re exploring, trying things out, finding out what works for you and what does not. The camera that can serve you best is the one that is flexible enough to shoot sports, events, landscapes, travel—anything you can imagine photographing.

A photographer, who is very tightly focused on a style of work such as street journalism or infrared, will find a natural counterpart in the Leica M8/M8.2; however, you will have to work within its vocabulary, its finite capabilities. If you are willing to do that, then you are going to be in for a photographic experience like none other, and the simple act of handling this gem of a tool will be something you will not forget. 

Oh, and of course, you’ll be about $3,000 - $5.000 poorer than the guy with the Canon.