I used to own a 50/1.5 Summarit, and one of my most-often-used current lenses is a 50/1.4 Canon.
The fact that the Canon is the one I kept should tell you something: By objective standards it's a much better-performing lens, especially at wide apertures. It's more like a modern lens in terms of its ability to produce uniformly sharp, contrasty images.
The Summarit has a more "vintage" look, with flarey highlights and soft corners; sometimes that's a very nice look, but make sure it's what you want. Another way of looking at this is to say that the Summarit has a definite personality that it stamps on your pictures, while the Canon pretty much keeps out of the way.
Here's a link to a picture in my gallery in which I think the Summarit's personality DID contribute to the effect; as you can see, though, the highlight flare is pretty dominant:
Link to gallery photo
One more thing about the Summarit: The one I owned seemed to be a bit dodgy in terms of construction for a Leica optic. The focusing ring barrel seemed to be quite thin -- probably in an effort to keep the weight down -- and if you squeezed it firmly while focusing, it would deform enough that you'd feel a definite "drag." Others have told me they experience no such effect with their Summarits, so maybe I just had a "thin-skinned" example, but...