Didnt say anything about being serious did I. There is Serious, and then there is a Killing Intent. Sasuke has a Killing Intent, while Naruto is just "serious". Sasuke would have killed Naruto if he didnt choose to use a different path then Itachi, while Naruto was only break all of Sasuke's bones. There is a difference, and it makes a difference. MMA fighters are serious, they dont fight with an intention to kill, Assassins fight with the intention to kill. That doesnt mean that the Assassin is stronger or a better fighter, it just means that he/she is willing to go that much further to win. Sasuke would be the Assassin, Naruto would be the MMA fighter. Different skillsets, difference roles, even though they are probably equal in fighting ability.