I’ve been using Plex for over a decade and Jellyfin for a few years, IMO Plex is the better of the two, but that’s only because it’s an actual company not an open source project like Jellyfin is.
The fact that Jellyfin is written in .Net makes it a pain in the ass to install on Linux (if you’re not using some sort of containerized installation) and it always floods the logs with gigantic stack traces anytime something errors out, and it’s usually only helpful to the devs, not the end user.