Wow, lots of info passed by here! Congrats on the Zorki-6!
When I bought my Fed-3b I compared it to a couple of Zorkis (I don't recall which model). The Zorki view finder was spacious and open when compared to the Fed's tunnel. But at least on these samples I felt the Fed RF patch was clearer. In retrospect I wish I'd bought the Zorki, probably, as the RF wasn't that bad and I sure liked the viewfinder. These were cameras sold by a repairman at a camera show, so all were in good operational shape.
Apart from sun-pinhole in the cloth shutter, there's another thing to look out for. This happened with my Fed: the cloth itself became worn where it wraps around a roller, to the degree I got light leaks, but only when the shutter was cocked. I guess that would be the first curtain. With the shutter not cocked the second curtain was solid and covered everything.
The fix is easy: paint the curtain with some fabric paint or similar.
Overall the LTM mount is much easier to use and so on. The Contax/Kiev can be an ordeal to get any lens other than the normal one mounted. But like Steve says, I also find the Kiev mount lenses seem better than their LTM counterparts - I've read somewhere that maybe it is the cams required that are simpler in the Contax design. Who knows?
Overall, I like my Kiev better than I like my Fed, in fact I'd be happy shooting just the Kiev if I had no other cameras. But someday I'd like to try a nice Zorki...