I currently use and host both, plex while non free is more friendly to less advanced users and has native iOS and apple tv apps (which jellyfin does not (the jellyfin ios app is a webview and dosen't always behave well for me))
There are 3rd party apps for jellyfin on apple tv, but it's just not as smooth of an experience as plex.
I'm hoping that jellyfin will push plex to get better, as some of the most requested features for plex have gone unanswered for years, which is quite frustrating for software that is paid.
Have a look at Infuse, it works with Plex, Emby/Jellyfin, and possibly SMB. It’s one of the best and high quality apps I’ve encountered on the Apple TV, and one of the best video players period. For me it completely eliminates the need for transcoding, it plays everything.
There’s also a bare bones native Jellyfin app for tv/iOS called SwiftFin, but it’s currently only (publicly) available in TestFlight.
There are 3rd party apps for jellyfin on apple tv, but it's just not as smooth of an experience as plex.
I'm hoping that jellyfin will push plex to get better, as some of the most requested features for plex have gone unanswered for years, which is quite frustrating for software that is paid.