hmm...sorry to join so late, but here's my 2 cents.
Hikki in the US?
I'm also worried about Hikki making it in the US cuz no Japanese artist (or Asian) has succeeded there yet (that i know of). But her Kingdom Hearts popularity will at least draw the gamers
Ayu in UK:
I think Ayu would have a chance in UK cuz i remember a while back, one of her songs made it into the top 20 there. I was really impressed when i heard that.
Looks-wise, Ayu VS Hikki in US:
i think Ayu would win cuz she stands out w/ her dyed hair and various fashion styles. I think Hikki would look like just another Asian to US people. I've heard so many times, "Your Asian singers look all alike". It really ticks me off to hear that, but that is reality, unfortunately
Ayu's voice VS Hikki's voice:
yeah, i'm not a fan of deep voice either - i tend to prefer high-pitched voice cuz it sounds more cute
But i guess i got used to it, so i now i don't mind Hikki's voice, or Koyanagi Yuki & Kuraki Mai. But as much as I love Ayu, her live voice is not that good...it crackles, or whatever you call it...but as my username says, that doesn't change the fact that "I love Ayu"!
that's all i have to say...i hope i didn't offend anyone...