I get asked by clients about the best way to send post-interview thank you notes - email versus snail mail. I strongly advise sending hand-written notes. Email can be viewed as less labor-intensive or thoughtful.
You can still get the notes in quickly: Put them in the mail the morning after you've completed your interview. I've even had applicants tell me they bring blank thank you notes to the interview day, complete the cards after their interviews, and leave them with the administrative assistant.