Annika is preferable, I think, because of its much more beautiful, classical spelling. But I suppose if the mother was Dutch and the father German, it would make sense as it would be a sort of compromise.
My name is Anneka, spelled exactly the same way as shown above. Parents out there who are considering this name, please, please think seriously about spelling it with an I instead of an E. Through my own experiences, I have come to learn that nearly everyone who looks at this name says it stressing the long E. It's very annoying and I want to warn parents that it will be a situation that they will encounter throughout their life!