Contrarian view: It depends. United objectively has more nonstops from SFO than Alaska. If you're traveling for business and value nonstops above elite benefits, then United may be a better option for at least some of your travel.

Also, just because you're flying United doesn't mean that United is the best StarAlliance frequent flier program to use. For example, it's still relatively easy to earn status on Aegean provided you can include some flights on their metal in your travel plans. This can get you perks like lounge access on domestic flights that would otherwise be unavailable.

You might split the difference. It's easier to earn elite status on Alaska than on United, and you can get many of the benefits of lower tier United status with a United credit card. Given that, you might want to pick up one of the cards, fly Alaska where the schedule makes sense (likely earning elite status), and fly United where the schedule makes sense (likely not earning elite status, but this doesn't really matter).