I tried both a while back, and settled on Plex, as it seemed more fully featured, plus the there's an app for it on my TV, so the UI and UX were better too.
The Jellyfin Android TV app is much faster and more stable the Plex's. For me the Plex app keeps bogging down and crashing several times while I browse my library.
That's a huge plus, however sadly Jellyfin doesn't handle subtitles well, and I really dislike the lack of a quick "skip forward" action. So I tend to still use Plex.
What did you find JellyFin had, that Plex didn't?