I have both (they both can coexist peacefully on the same library). I use jellyfin for any watching on my phone or computer.

However, where jellyfin still really kind of falls apart is when casting to my Chromecast. Controls don’t work, subtitles are unpredictable or missing, and it’s just generally a mess.

So I use Plex for casting, and jellyfin for everything else. I bought a Plex lifetime pass ages ago, so it’s an easy call to just have them both running.