The short answer: Keep the one that works the best for you. And by that I mean the one that combines the level of reliability you can live with, the specs you want, and the appearance you like.
The longer answer:
I have pretty much done this; from a large-ish collection of FSU cameras to one user.
I have owned and used: FED 2, 3, 4, and 5; Kievs 2, 4, 4a (and 60); Zorkis 3m, 4, and 10; Agat 18k. And possibly one or two others. And am now down to the FED 4.
I didn't own these all at the same time or I would have made a different choice. I like a well adjusted Zorki 3m the best of all the FSU cameras I've used and may get another. If I do then the FED 4 will find another owner.
I enjoyed shooting the Kievs but the lens choice was/is a bit limited for my budget. By going with the LTM mount cameras, I have lots of good, cheap-ish lens choices available.
To be honest, I probably wouldn't have kept any of my FSU cameras if I didn't like the J-12 lens so well.
Rob
(hope this post wasn't too far off topic🙄😀)