Both are really good, it's just that an epic adventure to save the whole world from the filthy fire nation and her warmongering leader is IMO much more interesting than "just" fulfilling one's duty as an Avatar and save Republican City.
In every season of TLA you could see growth, progress, all seasons were tied together, you could really look at it as beginning, middle and end. TLOK not so much, still very enjoyable nonetheless.
Btw, I loved Zuko's and Iroh's voices. And to be honest other main characters of the fire nation weren't really bad at all (Azula, Ozai?).
Anyways, Attack On Titan is the best anime I've ever seen. But after that The Last Airbender/Legend Of Korra all the way, especially after that crap Kishi fed us -_-







