Definitely take Ahri. She is really viable IMHO. If u can hit at least 3/4 of your skills ur dmg is pretty high, because every spell, expect taunt, is AOE. Ulti is giving her really great mobility and roaming power, so she can basicallly gank 24/7 after lvl 6. But she falls of quiet well if you cant land your skills. But she is a champ with high risk-high reward and she is far better then fiora.
Fiora must be really ahead to do some decent damage after her ulti was nerfed. She has no CC which is IMHO the most important part of current meta. And her dodge isnt at much use at all. At all she isnt brining any utility to team and there are many bruisers which are doing her work better.