It's always better to make your preference known than to act neutral and be disappointed if accepted to the other campus. I know in the past people have called to update admissions after having marked that they had no preference, but I'm not sure if this was pre- or post-interview. Either way, it can't hurt for them to have it on record that you are enthusiastic about one over the other for a concrete reason. Just be respectful and polite and you'll be fine (good rule in general)
I sent an email to admissions post-interview. Which helped me provide more nuanced reasons why I preferred one over the other. Admissions added the note to the file. Strike a polite and grateful tone with this one...