Although Sindre seems to have slowed down development to a point where one could argue that the lack of some 'modern' features feels outdated...
I think Subsonic is still better than Plex (regarding music), since it handles a music database [b]much[/b] better, leaner and faster.
Plex feels slow to me, and I have to take care of two 'databases', the ID-tags of individual files and the Plex-DB. A full db-update in Plex takes 2 days, and I have to check if every artist/album/song-match is correct.
I think Subsonics online-match with Last.FM is much smoother and faster, and works better. Overall, if your tags are fine, Subsonic is fine.