Is it just me, or are all the Zutara shippers people who like Zuko better than Aang? And are all the Kataangers people who like Aang better than Zuko? Not only does this match with what several Zutarians and Kataangers have said on the subject, but it also makes sense - it is only natural that people will think that the romance involving the character they prefer is the better, more likely one - as the old saying goes: "People see what they want to see".
So let's do a quick round off with who you like better as a character - Zuko or Aang, and which ship you like better - Kataang or Zutara.
I'll go first. I like Aang's character better. Zuko's story was great, and I love his character, but just not as much as I like Aang's.
So far, the theory is holding out then.