I just picked up a Pentax MX with a 50mm f1.4, and Leica M3 and did a side by side comparison in this very dark living room. Both are pretty much best in class in terms of viewfinders. The MX finder with f1.4 lens is brighter to look through, but the rangefinder is definitely easier to focus in the darker parts of the room, on subjects where there are no hard edges to use the split-image section of the SLR finder. However, in the lit areas of the room [still pretty dark] it evens out a lot more, and it's easier to focus the SLR on textured subjects like an expanse of fabric as the microprism ring in the finder makes that a lot more obvious than the rangefinder does. The SLR will also focus a lot closer.

I'd expect I could nail accurate focus with the M3 more often than the MX for the sorts of subjects [people] I'd want to be shooting in low light, and I find myself hunting for focus a little less with the rangefinder. I'd still be happy enough shooting the MX with the f1.4 in any situation where there was enough light for me to hand-hold it, but the rangefinder would have preference.

N.B. the MX has a much higher viewfinder magnification than most SLRs, and that's with an f1.4 lens. If the lens was a stop or two slower, or the finder magnification/brightness a bit less, the rangefinder would have a significant advantage. I'd definitely prefer the rangefinder with wider lenses, too.

I've had much better success shooting at small gigs and social events with a rangefinder (or, to be fair, with a Hexar) than I have with an SLR.

On the other hand, I don't think you'll see much real-world quality difference with the lenses. Good quality fast primes in normal focal lengths -- 35mm, 50mm, etc -- are going to be excellent on both rangefinder and SLR systems.