I run both concurrently. I have a plex pass from way back when, maybe a decade or more.
What plex is now is not what it once was. Trying to socialize viewing habits, opting in by default to analysis, ads, reviews, and sharing that info has gone too far. Plex also works on these features such as discovery which benefits them, instead of open bugs.
That us why I can't recommend it.
As for a feature comparison. Jellyfin is snappier, and faster. Plex is more detailed in their interface, and has better Metadata. Jellyfin sometimes doesn't restart where I left off. Jellyfin is much, much better on mobile devices, but has less clients for tv's. Jellyfin doesn't rely on any server but my own, where plex wants to authenticate with thier own servers and ask for accounts (and money) to have full functionality. Jellyfin always downloads to a client. Plex...might. Plex has better handling of multiple streams in one file.