firstly................Hikki wins, for me!
I'm not sure if ms. ayu is more popular than hikki outside japan. I hear more of hikki here and she has already penetrated the international music scene. In terms of image,i think ayu's image is better. If only hikki maintained her cute japanese girl image where she wears short hair, she would have been more of a bombshell like ayu. However i'm not much on ayu's image anyway coz its too girly for me. sorry ayu fans, wehehe. On music, i haven't heard most of ayus songs so i can't tell. But i like hikki's voice more. I think that hikki should be the queen of jpop not only becoz she's a prodigy but also because she has a very unique way of reaching through the listener's heart. Her moods reflect all the spectrum of jpop and at the same time she creates her own genre with it.