I use Plex because the whole experience looks and feels more polished than Jellyfin (that’s personal but even the name Jellyfin irks me, I hate jellyfish), which is something I value more than openness. Also I bought the Plex lifetime a few years ago during Covid and at that time Jellyfin was nowhere near Plex in terms of user friendliness, Plex already had a TV app, a mobile app with ability to download locally, a feature to share access to anyone with a Plex account, automatic metadata update, etc. I don’t know if Jellyfin has all that even today.

If I were to make a choice today I think my first choice would still be Plex.