I think most people fly their own airline (at least top elites) because it is more convenient.. especially if you are in PHX, CLT, PHL, PIT, LAS you will fly more US.. if you were DEN, ORD, LAX, SFO, you would probably fly more UA. I choose on ease of travel and perks. I would rather be in F on a A321 then in E+ on TED. But I rather be on TED in E+ over Coach in US 733. So it depends. But I usually always get upgraded, and enjoy F over E+. I prefer the zone of privacy.. (buffer between me and my neighbor) wider seat (I am fit, but prefer to be able to feel more confortable, and I do enjoyt he extra legroom (and there is some no matter how thick the seat).

So I mostly fly US now (only choose UA before when vs US RJ's).. so that will probably stay the same.