That's a hard one. While Naruto was hated by everybody in the village and treated like dirt, Sasuke's own older brother slaughtered his entire clan, including his parents, at a very young age, and lived on hating his brother and thinking only of revenge, hurting him more and more as it continued. Naruto never had anybody to begin with, but Sasuke had people that he cared about once but then lost them all in one night. Everybody wanted to be friends with Sasuke, even Naruto at one point, but the only thing that Sasuke cared about was his revenge and becoming stronger so that he can carry out that revenge. Those aren't exactly thoughts somebody his age should have. But Naruto had to live with being hated by everybody for a reason he didn't even know until he was 12 years old. Sasuke suffered the pain of great loss and lived blinded with revenge, even more so now than ever before, and Naruto suffered unwanted loneliness all his life, all the while being treated like nothing. Either way, they were both incredibly lonely. They both suffered similar pains, yet different ones. It's very difficult to say who suffered more. I think that it's both.