OTOH, supply of lenses will be better for the rangefinder; the Leica-fit world is your oyster, whereas 39-mm SLR lenses aren't so easy to find.
On the other other hand, once you fit those lenses, you'll see what the lens sees without needing an accessory viewfinder; I'm back to prefering an SLR.
I like my 3M but there is very little choice in lenses. However, those lenses are pretty good. I think I like it more now than when I bought it 40 years ago: it slows me down, and that's a good fit with the current cost of film photography. It's a very pleasant thing to take a photo with.
But without knowing what other bodies you have, I'd probably avoid the 3M unless I knew I could pick up a Tair 11 and Mir-1 at some point. Again, there's no lens available for the 3M that isn't a better purchase in the M42 mount. I love my 3M, but I'm not sure I'd buy one today.
3M = non-returning mirror, dim viewfinder with poor frame coverage (67%), poor focusing screen and more difficult reparability if damaged, when compared to your average rangefinder. Yes, SLR is generally better than the rangefinder, but early Soviet (and arguably - even some later) SLR is a half-baked product and I personally would pick Zorki.
I'd choose Zorki-6, because I already have around a half-dozen Zenit cameras, including an early Zenit-E with 39 mm lens mount, whereas I don't already own a Zorki-6.