I would like to point out one key difference between Zuko and Aang. Take it as you will.
First off, I hear this a lot about Aang: "He should have killed Ozai!" and then, in response: "He couldn't kill Ozai because valuing life is in his
blood, it goes against his very being to consider harming another. If he had killed Ozai he wouldn't have stayed true to himself."
Now, I agree with this defense of Aang. He is kind, and good, and loving, and probably mentally incapable of hurting someone else, much less killing them. (Also killing Ozai would have had certain political ramifications but WE WON'T GET INTO THAT. Plz excuse my polysci nerdiness.)
But what I think about often is the fact that Zuko overcame the one thing that was in
his blood, that was part of
his being, that was as important to himself as non-violence is to Aang. This thing is faith in the goodness and righteousness of the Fire Nation.
As is clear from the episode when Aang goes to school, Fire Nation children are taught from the beginning that the Fire Nation is the greatest nation in the world. We can also glean from later episodes that all children are taught that, through the war, the Fire Nation is trying to spread its prosperity and "modernity." These lessons were as ingrained in Zuko as non-violence was ingrained in Aang.
I would also like to point out the implications of an Agni Kai in Fire Nation culture. Because even the most important issues are decided by Agni Kai (ex: Azula and Zuko Agni Kai over who gets to be post-Ozai Firelord), this suggest that the Fire Nation believes in a survival of the fittest/Manifest destiny sort of thing. In other words, the ones who are
meant to prevail,
will prevail. This, too, was integral to Zuko's upbringing, and allowed him to believe that because the Fire Nation was winning the war, it was
meant to win the war. (Sidenote: For those of you that don't know, manifest destiny was also used as justification by the US for driving Native Americans from their homelands.)
There is also the matter of Zuko's birthright. As first born, he was raised as the heir to the throne, and I think everyone can agree on how important Zuko's birthright was to him. Again, all of this was as deeply embedded in Zuko's character as non-violence was in Aang.
But Zuko overcame all of this when he left the Fire Nation and joined the GAang.Now, obviously the values Zuko with which Zuko was raised have less "big picture" merit than Aang's nonviolent values. HOWEVER, this does not mean it was any easier for Zuko to overcome his values than it would have been for Aang. But he DID overcome them, whereas Aang wasn't even capable of
thinking about killing Ozai (he kept looking for ways to avoid it).
In the end, these are my opinions on Aang and Zuko's respective storylines:
Aang's plot was a coming-of-age story. He started out irresponsible and afraid, and eventually came to accept his duty as Avatar. It is a very universal theme, but it is not unique.
Zuko's was more than just a coming-of-age story. He not only grew up, but in doing so essentially re-educated himself. He abandoned EVERYTHING--his old beliefs, his birthright, his
nation--for the sake of the betterment of the world. The Zuko at the end of the series is completely unrecognizable from the Zuko at the beginning, whereas Aang is...pretty much the same.
So in regards to the Aang vs. Zuko question, I love both characters. But I find Zuko's story more remarkable, more compelling, and much more powerful.