Both are fun! I usually end up with one new fandom and one closed fandom to obsess over every few months or so, and both are great. The excitement of the open canon is excellent, though usually there isn't a lot of fics at first. This is where the closed canon comes in! Usually for that one, there's a ton of fanfic built up, and I can read it while waiting for new fics to show up in the open canon.