I'm biased since I have never played Viktor. However as an Ahri main, I believe that Ahri is much better. She is hard to play, but not that hard like Riven or even Yasuo. Some players need to know that if they don't have their ult, they should be very careful. You just need to know when to get in the fight or not. In any case, I usually go q,w,e and level up r whenever possible. Once you get
it's like "gg get fucked enemy team." Everyone bans Zed for several reasons as well as Yasuo. But when I play Ahri mid, you can outplay your opponents, especially when you land a charm on them.
In short, Ahri is an immensely fun and semi-challenging champion to play.