Sasuke Uchiha and Naruto Uzumaki are two of the core characters in Masashi Kishimoto's Naruto. Kishimoto established a rivalry between the two from the very beginning of the story, with Sasuke being everything that Naruto wanted to be, and with time, Sasuke coming to envy the rate at which the latter progressed. Although rivals, the two often pushed each other to improve with their healthy competition.
While the way in which Naruto and Sasuke got stronger was certainly very different, the two managed to keep up with each other. By the end of the story, both played a major part in ending the Fourth Great Ninja War and were widely hailed as war heroes. Although Sasuke was certainly weaker than Naruto after the end of the Fourth Great Ninja War, due to the latter getting another half of Kurama, he managed to make up for the difference during the blank period by training himself to reach the same level as the Seventh Hokage. In Boruto, the two are often regarded to be equal to each other in strength. However, recent events could finally have shifted the dynamic in Sasuke's favor.
How Naruto And Sasuke Progressed Over The Years
Naruto and Sasuke's growth throughout the series mirrored that of Indra Otsutsuki and Asura Otsutsuki, which makes sense since they were the reincarnations of those characters. While Sasuke was a born genius and grew extremely fast at a level that shocked even Kakashi Hatake, Naruto struggled with almost everything and was reliant on those around him. It was only through love that Naruto's powers truly awoke and thanks to all the help that he received, he managed to become a recognized shinobi.
During the Chunin Exams arc, Sasuke was trained by Kakashi Hatake while Naruto learned quite a few things under Jiraiya himself. Although he started as weaker than Sasuke, he ended the arc as being stronger of the two, illustrated by the fact that he defeated the same Gaara that Sasuke struggled against. Naruto's growing reputation frustrated Sasuke to the point where he had to go out seeking more power.
After leaving the village and clashing with Naruto at the Valley of the End for the first time, it was quite clear that Sasuke started the fight being the weaker of the two, however, as the fight went on, the powers of the Cursed Seal emerged. What's more, Sasuke's Sharingan fully matured in the same fight, giving him the edge over Naruto and allowing him to win.
Spending over two years under Orochimaru, Sasuke further increased the distance between himself and Naruto, so much so that Sakura theorized Orochimaru was supplementing his body with something that drastically increased his level of skill. Even though Naruto trained quite hard himself during the time skip, it is safe to say that he was nowhere close to Sasuke's level at the beginning of the second half of the story.
However, his training under Kakashi certainly did help him. Not only did Naruto improve his usage of jutsu in battle, but training with shadow clones, as suggested by Kakashi, gave him years worth of experience in an incredibly short amount of time. Against Kakuzu of the Akatsuki, Naruto was a different beast entirely, evident from the fact that he didn't just rush into the fight like he might have done in the past. Instead, Naruto gauged his opponents and tested him well with his shadow clones, figured out his strength and weaknesses, and even calculated the range of his attacks, which was unprecedented. With the Rasenshuriken, it is safe to say that Naruto closed the gap very well.
Following this period, Sasuke gained the Mangekyo Sharingan and Naruto learned the Sage Mode, however, Naruto certainly looked stronger than him, given that he defeated Pain while Sasuke struggled a lot against Killer Bee. Eventually, the Five Kage Summit proved to Sasuke that he needed more power, and thus came the next power-up, the Eternal Mangekyo Sharingan.
To match this, Masashi Kishimoto gave Naruto the Kurama Chakra Mode, and against Madara Uchiha, the two were, more or less, equal. When Sasuke received his final power-up, the Rinnegan, Naruto received the Six Paths Sage Mode, and with that Kishimoto balanced the two out.
Why Sasuke Might Be Stronger Now
While it is true that Naruto and Sasuke were, more or less relative towards the end of the story, the former quickly gained a massive advantage thanks to the other half of Kurama that he received after the fight against Sasuke. To make up for this, Sasuke had to train his body to reach the same level and in doing so, he strengthened himself further. For Sasuke, this holds tremendous importance as, unlike Naruto, he didn't just get an extra Tailed Beast to get stronger. Instead, Sasuke had to train himself, meaning even if he lost the Rinnegan, the training that he underwent to match Naruto isn't lost.
On the other hand, Naruto completely lost what made him special and all he has right now is the power of Sage Mode to fall back on. The very same Sage Mode that Masashi Kishimoto paralleled with Sasuke's Mangekyo Sharingan. The interesting thing is, Sasuke still has access to his Eternal Mangekyo Sharingan, a power that needed Naruto to have Kurama's chakra to match in the Fourth Great Ninja War.
While it is true that Sasuke has just one eye, it doesn't make as much of a difference as Sasuke can use both Amaterasu and Kagutsuchi with the same eye. What's more, Madara Uchiha fought all the Tailed Beasts and defeated them without any eyes in the Fourth Great Ninja War and Sasuke still has one. At the same time, Madara summoned the Full-Body Susano'o while his real eyes were in Obito's possession, and Shisui Uchiha did something similar even after having one of his eyes stolen by Danzo Shimura. It is safe to assume that Sasuke still has access to the Full-Body Susano'o and, once again, this power was matched by Naruto using his Tailed Beast Avatar, which he doesn't have anymore.
Unless Naruto's Sage Mode has received new powers that fans haven't yet been made aware of, Sasuke clearly holds the advantage, with powers such as Amaterasu, Kagutsuchi, and even Susano'o at his disposal. The Seventh Hokage, who was clearly the strongest in Konohagakure until recently, might well be behind Sasuke. Naruto will once again need to train himself if he hopes to catch up. Fans probably don't need to worry about him, as he's risen to the challenge on numerous occasions in the past.