Either Naruto or Sasuke goes evil because a battle over the title of Hokage-sama is nice but it isn't as emotional and it isn't till one of them dies.
Sasuke:he can go evil just like Madara became evil. He can be rejected by the leaf after all he has done, even after he saved the world by ending edo tensei.
Naruto: he can become evil too if a lot of people around him die. He nearly lost it when Neji died, it was because Hinata snapped him out of it. If Kakashi, Hinata and a few more die he will side with Obito, and he would turn out to be a second Obito, because those two are like each other.
My guess is that Naruto will become evil and a lot of you think that is impossible but just a chapter ago Sasuke said he wanted to become Hokage and F0ddera-chan is getting a powerup(OMG impossible!!!).
And for those who doubt my theories go and check this thread I made some time ago. It states that Sasuke and Naruto might fight each other over the title of the Hokage which was not too far off if you look at it now.
You heard it here first guys!