I am not an experienced magit user, (because I don't use emacs) so I can't really speak to this directly, but I can tell you what I understand to be the case.
Here's the motivation for jj, described by its creator (this will be on the FAQ in the next release:
The project started as an experiment with the idea of representing the working copy by a regular commit. I (@martinvonz) considered how this feature would impact the Git CLI if it were added to Git. My conclusion was that it would effectively result in deprecating most existing Git commands and flags in favor of new commands and flags, especially considering I wanted to also support revsets. This seemed unlikely to be accepted by the Git project.
Fundamentally, jj is a different VCS, not just a UI layer on top of git. And so there's a lot of differences, but they sort of sound more generic than specifically "vs what magit gives you."
I don't have time to be more lengthy at this exact moment, but I'll be curious to hear what others say, and I can come back and say more later if you're curious.
Here's the motivation for jj, described by its creator (this will be on the FAQ in the next release:
The project started as an experiment with the idea of representing the working copy by a regular commit. I (@martinvonz) considered how this feature would impact the Git CLI if it were added to Git. My conclusion was that it would effectively result in deprecating most existing Git commands and flags in favor of new commands and flags, especially considering I wanted to also support revsets. This seemed unlikely to be accepted by the Git project.
Fundamentally, jj is a different VCS, not just a UI layer on top of git. And so there's a lot of differences, but they sort of sound more generic than specifically "vs what magit gives you."
I don't have time to be more lengthy at this exact moment, but I'll be curious to hear what others say, and I can come back and say more later if you're curious.