Jellyfin is open source, based on Emby 2.4.X or something like that. It can do hardware encoding out of the box. With emby you have to pay for that. They now start with refactoring all the code, because apparently Emby code was really spagetti like.

There is an Jellyfin Dev in this thread. Ask him about the Emby Database design where DB and XML files where all mixed :)))

DL;DR: Not too much difference for the enduser right now. Might load a bit faster, because of website refactoring.