Simple. Naruto had nine little friends hanging out in his head that he could consult if he needed help, who knew their own powers perfectly. And Sasuke had no help since there was no one to consult about the deeper mysteries of the Rinnegan. It's like the difference between trying to figure out how to play a sport for the first time with no coach or teammates (Sasuke), and having a team of olympic coaches (Naruto and the Bijuu).
That's about the difference in their power. Naruto has help, Sasuke has to figure stuff out on his own. Kind of like a reversal from when they were younger.