I suspect the tiebreaker for the OP may be the international routes. From all the posts, domestic looks evenly balanced - I would choose UA because of E+ but I can see arguments for AA and DL (although the RDMs you earn on DL are, by all accounts, useless). If you want to redeem internationally, it seems that UA is still very competitive through its *A partners.

But international to Europe from SFO is not as clear-cut as the above suggests. Again, it depends where the OP is going. UA offers only LHR and FRA, with LHR (and some FRA) a complete lottery as to whether you can get lie-flat. LH to FRA or MUC in C is not good. LX to Zurich is excellent, but connections are not strong. Otherwise, nothing. However, if the OP goes with AA, it's only BA 2x daily to LHR but with guaranteed lie-flat and superb connections throughout Europe. Moreover, BA's hub at T5 in LHR is superb, whereas FRA (where LH hubs) is still a zoo. Not only that, on UA both LHR and FRA are reduced in frequency in the winter, so the position gets more limited.

Speaking personally, I'm with UA, I'm a 1K and I'm 2/3 of the way to MM. However, were I starting afresh, I'd probably go with AA/BA, notwithstanding the lack of E+. The constant changing of equipment (and thus loss of preferred seats) is the straw that is breaking this camel's back.