I think making sure to send a thank-you is more important than whether it comes by email or card, so I don't know that there's a wrong choice here. Just make sure that your thank-you is not a form letter (specific details from your conversation helps with this) and that your gratitude and sincerity are clear. Handwritten notes take time, which conveys sincerity. However, if you're not already accepted to this program, then the time it takes to receive a mailed thank-you may make it seem like you forgot to do it, which may have an impact on decisions.