It sounds like you would quickly hit the upper levels on any of the major american carriers. As you will most likely fly United TPAC I would go with them. I believe that you can apply SWUs on W fares, so using United might give you much better rides. This really holds for all the major carriers. If you fly them TPAC once a month and have other travels as well then you will quickly hit the top level, and most carriers take better care of their own than the others.