Do not send an Email thank you. It looks lazy. Much better to send a handwritten one on some nice, but bland stationary.
Your thank you should stress the positives you had in the interview:ie
Thank you for taking time to speak with me. I am now really excited about the possibilities involved with this project. I was particularly interested in the aspect of XYZ, as I have some skill in this area and am very interested to do more of this type of work...
It's one last selling point...use it.