IMHO, 35mm to 40mm is a big difference (I also don't like 50mm, so YMMV).

I find the J12 a good lens: I had 4 at one point, one was super-sharp on one side and soft on the other (decentering?), one was generally softer than the best 2, those 2 were equally good and not *too* far from my reference lenses.
For a Soviet made 1934(?) Zeiss design, that's pretty good!

The statements that the J12 performed better on Kiev bodies than LTM I find interesting... My own experience is mostly with LTM versions (briefly had the Kiev version, but didn't shoot much & didn't run any tests). Perhaps focus alignment problems? My own tests were mostly carried out on a Leica M body with an M39 adapter.