Keep in mind that as long as you keep your green card you are required to file income tax with the IRS every year or face fines up to 10,000. If you become a US citizen you will have to file taxes, report all Canadian income and assets as well as any accounts you may have signing authority on. This will be for the rest of your life. If you decide to give up your US citizenship, you will still have to file with the IRS for 10 years. I too am a green card holder and Canadian citizen and have decided that as soon as I come back to Canada I will gladly give them my green card at the border. After all, a "green card" is really a permanent resident card and I wont be a resident of the US any longer. If we ever decide to go back to the US I'll just file for status there again.