Because my handwriting really sucks, I prefer the more formal printed "thank yous".

The decision is ultimately yours based on your interview and what you feel the interviewer would view as appropriate. Also, when writing to thank the interviewer for their time and continued consideration of you for the open position, try to include one or two things that the interviewer stressed that you would like to have her keep in the front of her mind about you. IE: team player, able to command respect when heading a team project, different computer programs that are used by the company that you have extensive knowledge of.

If possible, see if you can obtain a business card with the interviewer's email address. Often the person interviewing won't have an actual business card, but the receptionist usually has business cards of the different executives and/or sales staff on her counter. If they have an email address, it is not dificult to figure out what the interviewer's would be. My experience is that the employees of the company are usually given an email address that incorporates their first initial and their last name @ the company's name.com. IE: ksmith @bakersinc.com. Sometimes they also incorporate the middle initial. Ie: kasmith @bakersinc.com. By surreptitiously snagging a few of the cards (one from each executive) you can usually detect the pattern.

Or you can go the easy route and just ask the receptionist. Or the interviewer if you feel brave and confident.