Eh, I think it depends.

In terms of display and content management, even how the client apps work, I'd say Jellyfin is far, far ahead.

Plex's only advantage (to me) would be remote access, and even then I'd say its just how accounts are handled. Which is only good when Plex auth is working properly, and an absolutely awful experience when its not.

I like having SSO in front of JF, I think its a much better experience. Now I just need to figure out how to get my kids to use SSO without a device... Which I'm leaning toward an NFC reader and a tag for each of them, maybe in a toy or something they can tap to a reader. TBD.