The main difference between Plex and Jellyfin is the network setup. Plex takes care of that for you, while you have to set it up your self with Jellyfin. Another difference is that Plex can combine content from multiple servers ( I think. I'm not a plex user, so I don't know for sure), while it will always be seperate servers in jellyfin.
Jellyfin will always have my heart though, because it's open source and not here to make money. Plex also have a reputation to show ads and other stuff from streaming services.
Yes, it's the first line in the excerpt. Not really sure why it needed to be repeated in a comment too. :)