My rangefinders are a Zorki IV and a Retina IIIC, rather than a Leica. I do have a Minolta X-370 SLR with a few good lenses, and haven't picked it up in several years; although I may try taking it out, minus the motor drive and lens kit, with just the 50mm Rokkor and see how it compares with my recent fascination with RF's.
In spite of the fact that the metering on the X-370 is lots easier (the Zorki has no meter, and the selenium cell on the Retina is dead; I use a handheld usually), I have taken to the viewfinder of both my RF's, primarily because it doesn't look the same as viewing a magnified image on a ground glass screen. which is what an SLR viewfinder is like (think mini LF ground glass). The only exception I make to this rule is with my TLR, where looking down into that well-shielded finder at the nice, bright square image is a real treat.
I guess I like the image quality through so-called 'rangefinder' viewfinders (even including point-and-shoot viewfinders, which technically aren't rangefinders.) The image somehow looks more "3-dimensional" to me (that doesn't make sense optically, of course, but that's what it feels like.) It's the direct optical view of the scene, even if it isn't exactly parallax corrected, that I like.
I think where I tend to do more precise compositions on a ground glass are medium and large formats, with which I tend to compose in a more formal style.
Ultimately, I think uninterrupted optical viewfinders are about temporal accuracy: the timing of a shot (important for street photography), whereas viewscreen/prism finders of SLR's are more about compositional accuracy. Time vs space.
~Joe