I'd say it depends on timing.
If you think that the person will be taking any kind of action that may impact your admission SOON, then send an email. If you believe there is time for a written note to reach that person, send the handwritten note. Physical mail is far more intimate, especially these days, then an email. Especially when you consider that person's inbox may be overflowing.
Did anyone get a hand written note from the person who interviewed you ?
I received one from Kellogg following my interview. Would responding to it by email suffice ? or should I send a thank you card or something ?