Katy. Seems more elegant and has more of a flair - and it reminds me of the 'What
Katy Did' books.
Katie seems more like a little girls' name, and I don't know why, but this spelling has always made me think of a watery, wimpy person.
To be honest I'm not crazy about
Katy spelt either way, though. Is it for
Katherine? How about
Kitty, maybe,
Kerry, or just
Kate?