Anna beats
Katherine, but
Catherine beats
Anna.
I wouldn't worry about the nn
Kathy at all. I knew a whole bunch of students and teachers alike in high school who didn't go by
Kathy. My mom and one of my fellow students went by
Kathy. The others went by
Kitty, Catie, Cat,
Cate, Kit-Kat,
Rin Tin Tin, etc. More than one went by just
Catherine (I'd say at least 4 or 5 of them). There are sooooo many possible nicknames for
Katherine. I'd pick one early on and run with it if you hate a specific one.
Anna is a beautiful name. It's not underused at all, but it's definitely classic and definitely not too plain. I love tons of
Anne names, including
Anna.