Amelia has always been more common. I think it's more recognizable to people.
I personally prefer
Emilia. I think I do because it sounds more youthful (I've never heard it on older people like
Amelia), and doesn't remind me of the word ameliorate. Neither does it remind me of
Emily, but people might be reminded of
Emily and think it's an elaboration of that - and that might turn them off it because
Emily has been so popular for so long.
- mirfak