I don't really know AssertK, last time I looked at it, it wasn't that great but I think they progressed quite a bit.
IMO the main advantage of Atrium compared to AssertK is that Atrium has better error reporting:
https://github.com/robstoll/atrium#examples
AssertK on the other hand can also be used for native which Atrium cannot yet (open a feature request if you would like to use it on native, so far I did not had this use case)
Cannot really tell you more, let me know in case you do a comparison, would be interesting