I'd argue that UA would likely be your best bet for a number of reasons:
- SFO/SJC is UA hub which combined with their extensive network throughout the US means more non-stops
- Updates can be had relatively inexpensive (we've been seen checkin offers of $249 USD for SFO - EWR route)
- UA operates 767/777/787 on many hub to hub and transcontinental service meaning lie-flat beds on those red eyes and better service
- More UA cubs nationwide (lounges of the domestic carriers are all about the same which is to say not that great)
- UA is member of *A meaning you'll have benefits of status (i.e. lounge, priority boarding, etc.) along with the most comprehensive route network in the world
- UA has the million miler program which is even more generous than the AA one which gives you and your companion Gold status at 1M miles
The main benefits I see with AS is that milage redemption can be more lucrative and upgrades would be a lot easier since SFO/SJC isn't a hub for AS. Another rub with AS is that everyone is leaving them. Whereas they could boast partner benefits with DL and AA 2 years ago (i.e. priority boarding and seat selection) none of that is true anymore and I suspect with VX joining them, there will be fewer airlines wanting to partner. There's also some uncertainty with the milage programs now that AS has purchased VX (airlines have been known to play around with these things post merger).
Those are my 2 cents.
Safe Travels,
James