well ayu is the definitely most famous and no.1 singer in japan... why is hikki more well known? because of her first love and she is able to sing in english so that she is more popular... in fact on the quality ayumi's the one who is better singer
... coz i dont suit to hikki's voice (maybe) and hikki's song just to sentimentalic (too ballads or strange)... ayu isnt as universal as hikki becoz ayu's song isnt always hit (i mean as universal as hikki) in USA that i know Ayu's most popular song is Boys & Girls... in my place the most popular song is Depend On You... in japan itself it is said that LOVE ~Destiny~