TN Visa on Corp to Corp contract

I have a Canadian incorp with 2 shareholders including me. We have a contract with US company where they need my services for 6 months to Start with. I will have to make frequent visits to US and may even have to stay there for extended period to perform the job. I get personally paid in Cdn$ by the above Canadian incorp. Do I need a TN Visa in this case or I come under B1 visa? If I need a TN visa, who gives the employment letter - the Canadian incorp or the US Company?

TN and the letter should be from the U.S. company. Let me know if you want a sample case, we do many of these.