Definitely
Sophia. I dunno why. I guess it just flows better in my opinion.
I don't know any Sophies or Sophias.
I think
Sophie is a name on its own, but I imagine some Sophias do use it as a nickname.
I pronounce it the first way.
I like
Sofie better than
Sophie and
Sophia better than
Sofia.
I think that's too much -ia in a row. I think
Alexandra Sophia sounds better.