I use jellyfin for video so I can’t compare it to Plex there.

In terms of music, Plex/Plexamp have several “features” that make the listening experience awesome. You can make smart playlists based on track ratings, artists, genres etc. I don’t believe smart playlists are available for jellyfin music.

Sonic analysis is another feature that jellyfin doesn’t have. Plex scans your music and matches tracks that are sonically similar and you can generate mixes/playlists from that data. I think you can also use AI if you have tidal.

In general Plexamp makes the listening experience more enjoyable. I can’t count how many times I’ve discovered songs in my library that I hadn’t appreciated before.

Plex’s Plexamp over any of the Jellyfin FOSS music apps. Bought Plex pass just for the amp.