In my opinion Sasuke's development through part I and the up until he defeated Dazno was spot on. He is the avenger, that is his archetype.
His development began going downhill once he defeated Danzo. He fought his brother and killed the main reason the Uchiha massacre even occurred, but after that his motives became murky and really undefined, to wanting to destroy the leaf/Naruto, to wanting to become hokage, to killing the 5 kage and becoming the worlds villain in order to unite them.
his "character" is easy to relate to for so many because they want to see a protagonist/antagonist suffer both physically and emotionally. Naruto had tough battles but he never questions his motives (at least not for longer than one chapter) we haven't really seen a "change" in Naruto emotionally as we have with Sasuke.
I think the main reason why people like Sasuke though is because he has a darker background than Naruto. And really has nothing to do with his development. When dealing with ninja more people expect a history like Sasuke's from a protagonist's perspective than being ordained to be the worlds preachy savior.
I like Naruto better than Sasuke, but I agree that Sasuke is better written than Naruto.
Pain, Danzo, and Itachi are the best written characters in my opinion