For living in Mexico; should I stick with a FMT and renew it every 6 months or apply for an FM3?

I am moving to Mexico early next year. I am a U.S. citizen. I have been to Mexico several times, but for no longer than 3 months. So I have had FMTs, or tourist visas. Since I will be living in Mexico for several years, should I get an FM3 or should I keep renewing my tourist visa? I will possibly be teaching English while I'm there. Also, what would I need to do to get a FM3?