Finally! An intellegent article about J-pop
I've always thought Ayu sounded more Japanese than Hikki when singing. Her voice always reminds me of old J-pop and enka. It could also be that Ayu's primary language is Japanese whereas I can only guess that Hikki uses and thinks in English more often. It's a very noticeable difference in the way they phrase the words of their songs.
I love this article! Most Ayu vs Hikki arguments about their vocal styles come down to "she-just-sings-better-you-teh-suck". It's refreshing to read about it from this point of view, without too much subjective bias or value judgment.