When comparing Plex vs Jellyfin, the Slant community recommends Jellyfin for most people. In the question“What is the best home media server?” Jellyfin is ranked 1st while Plex is ranked 2nd.
Specs
Ranked in these QuestionsQuestion Ranking
Pros
Pro Supported on many devices
It's pretty hard to find a NAS or STB that doesn't support Plex.
Pro Accessible via web browser
A modern web browser is enough to get everything Plex has to offer.
Pro Added lots of media databases
Metadata for media files will automatically be updated by using media databases. This provides posters, movie trailers, descriptions, information about actors, etc.
Pro Open source
Pro Completely private
Doesn't phone home.
Pro Easy to set up
Setting up tv series and movies take little effort to set-up.
Pro No hidden costs
It's FOSS and doesn't require you to spend money to do anything.
Pro Good emby fork
It is a fully open source fork of emby.
Pro Portable version
This version runs on systems with a .NET Core runtime.
Pro Less intensive/more flexible options for transcoding
More control over transcoding and seems to transcode far better on lower spec hardware (eg SBC) than other packages (eg Plex).
Pro Standalone, no third party servers
If you stream outside of your local network, there are no third party servers involved.
Pro Direct streaming
If you stream outside of your home network, it is a direct connection with no third party servers involved.
Cons
Con Needs registration
You need a PLEX account even if you use it on your local LAN.
Con Proprietary software
Unfree software.
Con No gapless audio-playback
Plex still does not support gapless playback of audio files such as mp3 or FLAC.
Con Playback error: This server is not powerful enough to convert video
More hardware requirements on Plex due to a useless/non-intuitive transcoder. When using lower spec hardware (eg SBC) Plex will not transcode or play HD+ videos. When compared to other packages (eg Jellyfin which transcodes nicely using FFMeg), Plex falls short on video playback and transcode settings to get videos to play (fastest/lightest option still fails).
Con Streaming outside of your home is complicated
Unless you enable unPnP which is a security risk, you have to manually port forward or setup a remote server yourself. And unless you know what you're doing, you might open your network to potential hackers. Make sure to read up on reverse proxies or how to set up a vpn with Jellyfin.
Con Some poor clients
The AndroidTV-client is terrible.
Con No free services
It has no free with ads services like Plex.
Con Less features than Emby
Con Slow web interface
For large media libraries, on lower spec hardware (eg SBC), images can take some time to initially load when changing pages/sections (v10.6.4).