Sims 3 Supernatural or Sims 3 Seasons?

I personally find supernatural as the better add on because it adds more and is just downright hilarious in some cases.

It really depends on your preference. Seasons is one of the "must have" EPs in my opinion, it just makes the game much more realistic and enjoyable, and each season adds many different options and interactions. Also Winter is so beautiful in game and so enjoyable, even though it might make your game a bit laggy.
On the other hand, Supernatural is a really nicely done EP, but I think it could have been done so much better, and to me it kind of seems that EA rushed over this EP. Even though it is supposed to be a fantasy EP and supposed to add unrealistic experience in your game, I think some of the things in it are just ridiculous and way too childish(Yes, I'm aware, that it is a game aimed at children/teens). The way the vampires and the werewolves were made in the game and the interactions are just way too plain. And then I didn't even mention the zombies or how annoying the fairy glowy wings are or the glowy vampire eyes and skin are. Although the objects and CAS items are beautifully done, and to this day one of my most preferred EA hair is from the Supernatural EP.
Again, it really depends if you rather prefer realistic or unrealistic content in your game.