I think Sora is faking his attitude for most of the story; I mean, when he first met Donald and Goofy, they basically told him that he needed to smile or they wouldn't take him with them. To a 14 year old who's world has metaphorically and literally been turned up-side-down at least, that means "Suck it up; we're not dealing with constant worry when we don't even know your friends. We'll help you, but shut up."
I also think that Roxas was the "more relateable" one because he didn't have to worry about his friends, and he didn't have to worry about anything really (all the things Sora had to and otherwise), since he didn't have a heart the way Sora had one. Even in his "own little world" (Virtual Twilight Town), he didn't have much to fret about. For the most part in Days, he was trying to do what he thought he was supposed to do (collect hearts, etc.), and then he worried about his friend. But for the most part of the story, he was just trying to exist the way he thought he could (as Roxas, not as Sora).
Both are pretty believable to me.
Either way, I like Sora more. His main drive is to find or protect his friends, whether it be Kairi and Riku, or the new friends he earns while looking for his two besties (when trying to stop the influx of Darkness and such). Where Roxas' main drive is more like protecting himself and trying to find out who he is.