Ahri or Renekton?

I never played Renekton but I enjoy playing Ahri because of her ability to burst champions down and her Ultimate can ensure her to dodge any attack including turret shots when dashed at the right times. It's really up to you because one is melee and the other is ranged.

is obviously the superior choice!

All you have to do is buy a Black Cleaver and then you deal a ton of damage like Phreak. I usually buy Black Cleaver, Maw of Mal (if needed) and a lifesteal item like Ravenous or BT. Whatever you do, don't build him glass cannon because then he will be useless.

Ahri just got nerfed hard, so I would go with ren, especially since he is getting a Visual Update (and possible rework?) soon.

This is a great question and I really think that its only a question you can answer. All champs come and go in a cycle that some call Flavor of the Month (FOTM). Meaning that while their are over hundreds of champs to choose from, there is always a pool that is currently being used. Right now you could say that assassins such as Ahri are in the current "meta" or flavor. I should follow that with saying Renekton is not an assassin but currently is played in this meta as a great top laner who is a lane bully and great teamfighter.

The main point you should consider is which lane you prefer to play and work on. These two champs are fantastic and played with great success (while also being loads of fun). I would personally recommend that you pick them both up sooner or later. But in the end its currently up to if you want to join this assassin meta with Ahri or powerhouse top with Renekton.