I guess I can pitch in here a bit, but please keep in mind, these are just MY opinions, if it sounds like I'm making sweeping statements I'll apologize in advance as its not my intention.
Anyway, I have a 5D(mkI) and a Leica M8. In terms of IQ, I would give an edge to the M8. The 5D, with its low AA filter, provides very sharp pictures. However, the M8 has no AA filter at all, thus providing an advantage. ISO wise, the M8 goes 160, 320, 640, 1250, 2500. 160 & 320 are beautiful. The latter 2 are just nasty. The 5D simply blows the M8 away in terms of high ISO, and from what I've seen, the M9 does not fare all that well against the 5D either.
Build quality, I'd give the edge to the Leica. It is fairly small, but quite hefty. There's very little plastic, and you just feel like you're holding a chunk of solid brass. However, it is definitely more comfortable to hold the 5D (or any DSLR, basically). But an added accessory (ThumbsUp) helps the M8 feel more comfortable than without, though still not as comfortable as the 5D.
Handling wise, I guess it depends on what you like. Even though the 5D is considered archaic these days, its still quite "fiddly" compared to the M8 for me. There's no autofocus, no metering selection, etc. on the M8. You just kinda turn it on and shoot. If you like simple, the Leica is it.
One thing you should take notice is the viewfinder between the two. For me, I've felt a bit constricted when looking through the 5D. I know, weird to say since I remember being WOWed by the 5D's viewfinder when I first looked through it. I guess its because with the Leica, you're not seeing through the lens, but a general viewfinder. This allows you to see quite a bit of what is outside of the frame, depending on the focal length. This gives me a feeling of openness.
In the end, a rangefinder and an SLR are just too different to compare. Definitely go out and at least hold one before you buy. See how you like looking through the viewfinder, focusing the rangefinder, changing the aperture/shutter speed. Hope this helps somewhat.