I sent a thank you letter to anyone that was involved in my interview. This included the program coordinator and the residents that interviewed me/ gave me a tour/ or took me out to dinner. This meant that I had to send 6-7 thank you letters for each program that interviewed at.
Realize that most programs will have a meeting at the end of the interview season and the residents, attendings, and the program director will go over each applicant that they interviewed and decide together where to place them on the rank list. Although the ultimate decision is by the program director, everyone in that meeting has a very strong say in where the applicants will end up on the rank list. Definitely, if your grades, letters of recs, board scores, and deans letter make your application weak then a thank you letter is not going to make that much of a difference. But if you got the interview that usually means that you are a strong candidate and the more people remember you in a positive fashion the better your chances. This includes the residents and attending that you talk to during your interview tour or during your lunch and or dinner with them.
I would stay away from email thank you letters. An email thank you letter might come across as you did not want to put in the effort to send a thank you letter and just wanted to get it over it.
Emails are great for post interview questions. I strongly encourage you to use it for that purpose. This will allow you to make a wise decision about a program and will let the program know that you are serious about them. 😕