I used both of them for a long time. Plex(amp) for music because it just works for streaming my collection everywhere I go and has a good UI. My own personal Spotify… And Jellyfin for everything else.
Now I built a new homelab server with a beefier AMD and proxmox, and decided to just switch to Plex for all my content. It has a better UI to my taste at least, but it also has these weird glitches sometimes where the video playback stutters if watching 4k material (where Jellyfin just worked).
I don’t know really. For music Plex is definitely worth the subscription price, but these weird playback issues put me to consider alternatives for video content.