If you are not going to do 100k per year, I would say that UA would be slightly better than AA, as you can use their E+, at least until they match AA and drop it, which I find as adequate on domestic and at least tolerable on international. (Though I have only had the NRT-HKG-NRT on UA international.) Otherwise, AA is still the easiest to UPG into business/first on both international and domestic travel, at the EXP level, given that eVP's have better (read lower) fare options and the unlimited domestic UPG's, which have been 99.9% (for me, only one DFW-SFO segment in the past 3 years-though some of that time, it was e-certs.)

Definitely, concentrate your flying into one program until you start all the MR activity to do multiple top elite status.