I've used both, jellyfin was useable, if at times a little clunky. plex is more polished however has its own tarnish added by the way the devs are pushing how you can use it.
Currently I'm running plex (wrongly) on the cloudron server, because despite having a GPU & working drivers in it, due to the way the docker system and jellyfin work together the performance was rubbish. One day I'll dedicate a different box to plex. I bought the lifetime license, so get all the niceities that seem more elusive on jellyfin.
I use a roku for one TV, and while my other smart TV (Sony) is much newer the built in app runs worse