Sasuke Reached Kage Level Before Naruto
Naruto only became a kage level ninja once he learned sage mode and fought Pain. This happened at chapter 430 when Naruto finally returned from Mt. Myoboku to fight Pain.
On the other hand, Sasuke achieved kage level fighting prowess after absorbing Orochimaru inside his body. This happened during chapter 345, almost 100 chapters before Naruto reached kage level.
The reason Hebi Sasuke is a kage level shinobi is because he fought and beat Deidara who managed to capture Gaara, a kage. He also managed to put up a good fight against Itachi who was way out of his league.
Pre sage mode Naruto, on the other hand, is no where near kage level, he never faced off against a kage level ninja in a 1 VS 1 and isn't capable of soloing, let alone putting up a good fight, against another kage level.
Now let us look at their skills:
Sasuke had superior intelligence, taijutsu, ninjutsu, genjutsu, speed, hand seals and was far more versatile than Naruto.
Before Naruto learnt sage mode, Sasuke was on a whole other level.