First off, a FSU camera was mass-produced and cheap and a Leica was neither of those (relatively speaking). That being so, you should expect that a Leica was made to a better standard. Also, Leicas tend to be looked after and FSUs do not (obviously that's a sweeping generalisation).
I have several Zorki 1 and FED 1 cameras and only one Leica, a IIIC. The Leica is certainly a little smoother in operation, it does just feel of higher quality with better cut gears and so on. That said, the gap is not huge and they all do their job just fine. This is based on my limited samples, it won't be universally true.
Most FSU lenses should work equally well on a Leica. There are some with potential problems, like the Jupiter 3 and Jupiter 9, which are not confined to being problematic with a Leica, specificially. Pre-war FED lenses were not made to standard lens-register spec and the mount threads may also be different - best avoided. I have used some Industar collapsibles and Jupiter 8, 11 and 12 on my IIIC without issues.
Overall, it's fair to say that a well-serviced and not abused FED or Zorki 1 isn't far short of a Leica II/III. The main point being about serviced and not abused or neglected, which really applies to either FSU or Leica.
Zorki: 1e (x2), 2C, 3M, 4, 4K, 5, 6
FED: NKVD, 1g, 2e
Kiev 4, 4A
Yashica Minister III
Zenith C, Zenit C, Zenit E
Minolta XG-M, XD-5
Panasonic Lumix G2, 14-42 Kit lens + 45-150mm
"The way to see by faith is to shut the eye of reason." - Benjamin Franklin