the only real downsides to both is the app ecosystem and "normie-friendlyness"
Plex does the remote access gubbins and lets you just go "app.plex.tv" for when you want remote access
Jellyfin means UPnP/Port forwarding and doing direct IP or custom DNS records
if you're only ever using it *at home* and don't care for remote access, Jellyfin is pretty rocksolid for dropping in as a replacement and the apps *should* locally discover the server on the network