Also check the forum for SFO advice on the same questions (keeping in mind that UA has a far larger presence and share at SFO).

In general I would recommend AA over UA if you plan to use your miles for premium travel, i.e. upgrades or One World business class travel. UA blocks redemption on its partners and in general blacks out international C award travel until shortly before departure. I think most would agree that award redemption is rarely an issue on AA.

Also I have found it easier to do paid upgrades on AA than UA. And don't discount the benefit of dealing with competent friendly US-based CSRs vs. the awful ICC outsource that staff UA's phone lines.

However, if you will be flying paid C transcon then you may prefer UA's P.S. service. Also if you will be flying in the economy cabin you may prefer UA's E+ seating to AA's (though I have rarely had difficulty securing an exit row on AA).