VX has a pretty decent product out of SFO and like AA a much newer/nicer terminal than UA. VX only has a points/mileage earning system though. No other frequent flyer benefits like status, upgrades, etc. to speak of if that is of importance to you.
Both AA and VX have much lighter schedules than UA out of SFO, however, VX does seem to have a schedule that keeps increasing as they try to build a bigger presence. Not sure if AA is adding or cutting at SFO.
If you do a lot of short-haul domestic then WN out of OAK has by far the most abundant schedule for going up and down the coast, etc. - even better than UA the last time I checked.
DL and US also are possibilities obviously but I know less about them so will let others comment.