This is a no brainer. If you're going to live in Dallas, go with AA. Was just at DFW yesterday and couldn't believe how many AA planes there were. I know it's their hub and headquarters, but wow, they had three terminals out of five and a few gates at the remaining two. I think you'll have pretty much non-stop access to the world by going with American.
With that said, if you were living in Houston I would probably say the same thing about United.