the meaning of strength.. and who's stronger

Once Sasuke got healed by Tsunade, first he was rather perplexed, not sure what to do, but soon all the memories of the encounter with his brother had overtaken him. Since in the end, all that was left was anger and wrath of not being able to do anything. Additionally he had to think of Naruto, who had become so much stronger since in the beginning, and that even Itachi was, due to whatever reason (after all Sasuke didn't know anything about the Kyuubi) only interested in Naruto.

Soon after, all feelings of inferiority welled up and got the better of him - not being able to hold them anymore, he gave in and thus challenged Naruto to a battle. Naruto didn't understand the sudden excitement and told him that he first should recover properly- but after Sasuke started provocing Naruto, he agreed and only answered "I was thinking about it too."

The two went upstairs to the roof of the hospital and started throwing each other insults right in the face.

Sasuke got even more upset, frustrated and lastly also confused, once he had seen Naruto had started laughing. Thereon Naruto responded only that he'd be excited too, finally being able to beat Sasuke right there. Sasuke just told him not to get too cocky- and so their overwhelming emotions got the better of them and the fight began.

After countless exchanges of kicks and punches, Naruto's kage bunshin no jutsu and Sasuke's fireball jutsu, the fight got even more heatedly and Naruto charged his Rasengan followed by Sasuke's Chidori. Ready to almost kill each other, Sasuke came down while Naruto jumped up in the same moment - and in the very last second, both got stopped by Kakashi. He threw them against two water tanks, which only made Sasuke smile once he'd seen how small the hole in the tank was that Naruto had made with his Rasengan. But as soon as he turned, he noticed that the entire back of Naruto's tank was destroyed - furious over his own lack of power, he ran away.

Generally I'd say this clash of those two was only a matter of time. Ever since the events at the Chuunin Exam where Naruto solidly proved his strength and revealed his power in front of Sasuke, Sasuke, not Naruto, must have felt somewhat inferior, seeing how much Naruto was able to pull off. Actually everything had originally started with the battle against Orochimaru, where Sasuke could only watch since he was unable to move because of his fears and trepidation.

Furthermore these feelings only got pushed even more during the fight with Gaara, when Naruto once for all fought in full strength, and again Sasuke was stranded alone in his own lack of power.

But the worst thing was yet to come - Itachi's reappearence gave the last impulse towards indignity and humiliation for Sasuke. After all those years of hard training, there was still such a huge pitch between the two brothers. Moreover Naruto also seemed only to distance himself from Sasuke, power-wise - it was only time for Sasuke to settle his feelings in such a fight. Not to mention Naruto saw this as his chance to finally get acknowledged by the other teen. At first Naruto was really excited as well, when Sasuke had challenged him; but as we should later get to know, in the end this was utterly different from the fight Naruto had wanted.

What really affected me in this chapter was the fact that it was so obvious to see even underneath Naruto's mask of not taking Sasuke "seriously", speaking only seeing him as a mere rival, lied an amount of deeper feelings which really seeked the respect of the other one. Naruto didn't care for the recognition of anyone else at that moment - all he cared for was Sasuke's. Standing equal with him was all that mattered.

Thus, as soon as the two stood on the roof top, ready to fight, Naruto immediately brought the forheadprotectors up. He wanted Sasuke to put it on no matter what, as a prove that they were fighting as equal Leaf ninjas. Sasuke then claimed this is what he meant with Naruto being cocky, since to him, they had never been equal. Naruto replied that he had never felt inferior to Sasuke and thus, forgetting all their surroundings and completely taken aback by their strong emotions that all seemed to be released at once, they didn't hesitate to go at each other with their strongest Jutsu's - Rasengan and Chidori.

Actually, this fight didn't bring any answer to the question they both had in mind. Of course, if we follow the water container theory, Naruto's Rasengan was more successful then Chidori, but putting that aside, the fight only left two confused boys behind, who both must have felt something was wrong. They were more than serious about beating the other one up and after all that had happened, it was certainly not a fight between teammates, nor friends. After Kakashi also talked some sense into Sasuke, he realised as well how important Naruto and also Sakura are to him, and that a jutsu like Chidori is used to protect people and certainly not against a friend.

Not to mention Naruto's feelings - he was the first who noticed and also told Sasuke, this was by all means, not what he intended nor wanted.

Anyway, I'm sure this wasn't what Sasuke wanted either, but since both are the type of having some kind of superior complex (although Naruto wanted only to be equal in the first line) and also constantly being pushed towards that kind of rivalry, it was only natural for their rivalry to reach such an extent sooner or later.

Furthermore I guess one could say primarily Sasuke got heated this far because he felt threatened by the immense growth of Naruto. Naruto's rapid development in strength must have caught his attention, and since his last encounter with Itachi proved him that he himself didn't progress as fast as he wanted to, he wasn't able to accept nor handle Naruto's fast improvement at all - which is why he was so desperate to prove himself he still was stronger than Naruto.

Found at Chapter 175, Episode 84/5