SLR's over emphasize focusing. The rangefinder viewfinder de emphasizes it. This is there drawback for this kind of street thing.
When you are wondering about, you have to react quickly, and so you have prefocus set and know what aperture you have so you know how much depth of field you have. IF something happens, camera comes up briefly and you take a picture. Sometimes you will have time to refocus befire you lift the camera, but even that will be general andwill rely on depth of field to a certain extent, because its guessing distances quickly.
The SLR you work the same way, and put the camera up to your eye, its out of focus or slightly out of focus, you are compelled to focus before you push the button, which means often you miss the picture.
Any other kind of shooting and the SLR would have the advantage in theory I suppose. Add to that a quieter shutter (for the shy shooters) and generally smaller in size, and the rangefinder is better for candid shots for me.