Diana or Fizz?

Want to get a mid lane assassin that isn't picked very often (at least not anymore for one of them) and can't pick. I've tried both of theme in this free week and can't decide, and I understand that Fizz just got substantially nerfed but I still think he does his job at eliminating one of the carries on the team and then getting out (He is an assassin now, not a 1v5er). I like them both equally and since they're both free I've tried them both but I can't decide. Who should I get, boards?