I slightly prefer
Hannah, probably because it's the "original" one - not the original in the world, but the original in my personal experience. The first spelling I learned.
I was trained to say HAHN-a for
Hanna because I worked with a woman whose name was that. Now I like it that way.
Hannah is still HAN-na to me, a sort of last-decade's trendy name that brings to mind
Daryl Hannah and
Hannah Montana ... and another person named
Hannah who I worked with, who was blond and sporty and professional but not dazzlingly intelligent.
I do think it's sort of too common. Not too common in general, but too common on people in a specific age group, to want to use it today. I wouldn't want to be named
Hannah today because if I were, when I turn 20 the name will suggest that I'm at least 30.
I think it'll do what
Karen and
Christine have done - continue to be used steadily, but not terribly frequently. The similarity to
Anna gives it some longevity IMO.
Hana looks exotic-ish to me. In a land called
Hana Lee... lol.
I think you can spell it however you like and insist on HAHN-a. I got used to calling
Hanna that, anyone can.