A short note written on the back of a $20 bill would be sufficient.
(Car Talk reference, for those not NPR listeners)
Seriously, you're overthinking this. Whether or not you write a thank you note or not is very unlikely to have any effect, at all, on the outcome. I receive email and written thank you notes all the time. Honestly, they basically go directly into the recycle bin. How many times has not getting a thank you note negatively impacted an application: Never. How many times has getting a thank you note affected a ranking either positively or negatively: exactly once. Someone sent a thank you card that was full of confetti. When we opened it, it went all over the floor. This was in the days of anthrax attacks through the mail, so it wasn't well received, was a mess to clean up, and seemed ridiculously unprofessional. And we already thought she was kinda weird.
So: do what seems "right" for you. Email or snail mail is fine. I personally feel like you're all doing us a favor by coming and interviewing, and we owe you thank you notes. But who has time for that ish?
P.S. I have this discussion with my program admin every year. I want to tell people specifically NOT to send thank you cards, to avoid all the mail. She likes getting them. So she wins.