Sending a thank you note will help you stand out as someone who is taking their job search seriously. Jobseekers who send notes are the ones who stand head and shoulder above the mill of jobless individuals.
I would use a specific thank you card with either an appreciative message on the front, or some famous artwork or photography of a nature scene. A neutral stance.
Within the card, the note just needs to be brief and to the point. A simple expression of gratitude to thank the interviewer for taking time our of his/her busy schedule to consider you for the position.