i'd say its a tie. naruto was alone from the start and was treated extremely bad by the village.
sasuke had come home to his brother slaughtering the entire clan, then his brother toyed with his head and used a genjutsu to show sasuke how everyone died... then he told him that he never cared about sasuke and that he was only using him. then after that he was alone and had no one at all... no friends, family, or even someone he liked, he was alone and hated the world for what happened to him.
both of them had their lives change when they were put on team seven, built a friendship from nothing but anger towards the other... both in the end are equal to the suffering that was put in their lives.
the way i look at it is:
(Sasuke) to have everything that you loved and cared about for many years taken from you and youre left with nothing but yourself is one of the hardest and most Painful things
to happen to a kid. also having to listen to others discuss that youre alone around you and spending all youre time by yourself, that shouldnt have happened to him.
(naruto) hated right from the start because of something sealed inside of him that isnt his fault and is being punished by the village for is also one of the hardest things but to also have others point at you and say bad things about you and telling their kids not to have anything to do with you is also one of the most painful things to happen to a kid.
its a shame that these things happened to both characters, naruto has been able to accept the things that were said and done to him. sasuke is still hurting from the things that he lost and found out about his brother, the suffering that sasuke still endures because of itachi's truth has pushed sasuke to hate and revenge. if it wasnt for iruka telling mizu(that guy who tricked naruto about the scroll) that "naruto isnt a beast he is....." while naruto overheard what was said, naruto would be doing the same thing sasuke is doing right now and bearing a grudge against the leaf.
even though i said its a tie, i still fall more onto sasuke suffering more. the stuff about itachi somewhat changes the scale on which their suffering weighs.
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(SPOILER)