This is a tough one, since I love both names, though if pushed, I'd go with
Katherine here, and I'd keep this exact spelling.
Katie is a nickname I like for
Katherine only, since the nickname for
Caitlin will always be
Cait to me (or Kait).
And, while I like other spellings of both
Katherine and
Caitlin, these are my favorites, so I'd not change the said spellings for either of the two.
Top 3 Names: ♀ Nikita, Amelia, & Teresa / ♂ Elliott, Bastian, & Nathan
Top 2 Combos: ♀ Nikita Ravenwood Zenobia / ♂ Elliott Wolfbrand Maximus