I've thought about it for a couple of days, and I can't think of a truly compelling reason to choose a rangefinder over an SLR. Supposing this were an irrevocable decision, I would say SLR every time. With the SLR you can use the entire range of macro to super-telephoto, zoom lenses, bellows, tilt-shift, stereo, etc. The way the SLR works is so versatile, there's really no comparison. Really the only shortcoming of the SLR is actually a shortcoming of the shutter, slow flash sync speeds, and is not limited to SLRs as quite a few rangefinders have focal plane shutters.
I enjoy rangefinders because of their variety, their history, and because they're fun to use. I like them because they're all a bit different, and most SLRs are cookie-cutter in comparison (excepting the Exaktas, Retina Reflex and some others). But if I want easy, fast and WYSWYG photography, I reach for the SLR.
This is not to say that rangefinders are inadequate in any way. They're just different. The lens selection is more limited, but in day-to-day use, it usually doesn't matter.