I do a business format letter.
To me, I would rather bit a little over-formal than not formal enough. If the manager is more old-fashioned and I send a hand-written note, I wouldn't want them to take that as being young and not as serious.
I do keep the business letter warm though, so it doesn't seem too "much".
"Dear Ms. ______:
Thank you for the opportunity to meet with you yesterday regarding the ______ position. I appreciate the time you and the members of your team spent with me."
I then briefly (a couple sentences) say why I am a great candidate for the job, incorporating something that came up during the interview.
I end it with:
"Please feel free to contact me if you require further information, and thank you again for your time and consideration.
I wouldn't do an email, but that's my personal opinion. If you put your letter in the mail the same or next day from your interview, they should get it within a day, and they usually haven't made a decision in that short of time.