Sasuke could form a Rasengan, but Chidori is more effective with his Sharingan. Likewise, Naruto could learn Chidori, but he's better off with Rasengan (since he can add elements to it or throw it). It's not like either character is defined by some A-rank jutsu... anyone with a brain knows Sasuke could copy Rasengan whenever he wants. Doesn't take anything away from Naruto or make Sasuke the most powerful. If Sasuke clashed against Naruto in a Rasengan vs. Rasengan fight instead of a Chidori vs. Rasengan fight, at most the outcome would be the same.