Not a lot of difference in FED and Zorki, although some believe the standard Zorki is better finished than the FED.
If you want a camera that is likely to be a shooter, without repairs I would go for the later models of both brands. The Zorki 4 and 4K are decent as is the Zorki 6, which has a hinged back and a longer base rangefinder. FED 2 is a decent camera although some are old enough that you may run into mechanical problems. I like my FED 3A which is a lot like the FED 2 and has know wind. The later FED 3s have lever wind.
I'm a big fan of the MIR (peace in Russian). It is a Zorki 4 without slow shutter speeds and allegedly a top speed of only 1/500. Actually the 1/1000 is there, it just isn't marked.
Russian Isaak S. Maizenberg, an authority of Soviet camera repair, said the the Mir was one of the most dependable cameras because it doesn't have the slow speeds (which most people dont use anyway. A real plus on the Mir is it has strap lugs which later Zorki 4s and 4Ks don't have. I have had two Mirs and both worked perfectly even though both looked pretty beat up. Mirs were only made from 1959 to 1961.
Despite being bad-mouthed by some, the FED 5 is a good user camera with lever film advance and an adjustable viewfinder diopter. They are the newest of the Soviet LTM rangerfinders and many of them are in like new condition. Plus they are really cheap.