After just about any meeting, successful or note, it never hurts to follow with a thank you note. This shows that you respect your client's time and even if your product does not suit you this time, your client knows that you are a courteous and professional colleague with whom he would like to do business again.
Email or postal mail? You know better; think about the customer's business culture. Usually the email is good especially if you are thanking something informal like time.
Your tone is also something you can evaluate. In case of doubt, more formal is better. You do not have to be cold or stuffy but be respectful.
Here are some examples that you can customize according to your own situation.
It was a pleasure to meet you yesterday. Your ideas on new sales approaches in our area have been insightful and have helped me a lot. Thank you for taking the time to meet me and share your thoughts.
————————— ————–  Dear Dr. Price,
Thank you for seeing me yesterday. I enjoyed the opportunity to discuss our new products with you. Your opinions are always useful and interesting. I hope the latest product will suit your needs.
It's always a pleasure to see you!
——————– ———————  Josh,
It was a pleasure to finally meet you today. Your research project looks very interesting and offers a compelling value proposition to mobile operators. As discussed, my team has done a tremendous job in the type of data collection that your research requires.
Please let me know if I can provide you with more details on how we will work on the project. talk to you soon.
—————————————–  Dear Paul,
Thank you for your time today. Jack and I enjoyed your contribution to our new business development strategy. I know you have a number of requests at the moment, so the fact that you have taken as much time as you to guide us is even more welcome.
We will be sure to come back with you when the plan is in place. finalized so that you can provide your seal of approval as well as your final comments.