If the OP would have asked 2 years ago, I would have said AA. Today I would say UA. IME, it's increasingly difficult to find TATL or TPAC availability on both carriers, but now especially on AA. Yes, you have to purchase a mid-fare on UA (W class or higher) but I can find more advanced inventory on UA than I can on AA for the routes I fly. And as chgodrew pointed out, UA's premium cabins are steady or expanding while AA's are steady or contracting.