As far as playing the game goes, henchmen and companions seem functionally almost identical. Companions just have some dialogues while henchmen don't.
From what I've read in other threads, adding henchmen seems very simple: just adding their data to .xls somewhere. Adding companions would involve placing them somewhere in the world and (ideally) giving them some interesting dialogues, so that would be more involved.
But spoiler alert, I really have no idea what I'm talking about.
