The OP's options from SFO will be:

- United (SFO-LAX, SNA or ONT). United has the best frequency and coach product for elites to LAX. SFO-SNA is very expensive and occasionally a regional jet. I'm not sure what UA offers on SFO-ONT. UA also has a fantastic frequent flyer program, particularly if you are interested in earning international premium cabin awards on non-US airlines (such as Singapore or Lufthansa). The big downsides to UA are very bad customer service (expect no more than a middle finger if something goes wrong - even if it's United's fault) and higher costs in terms of $100 change fees and higher fares.

- American (SFO-LAX or SNA). You're presumably familiar with the pros and cons of American so I won't rewrite them here. Just know that SFO-LAX has OK frequency and SFO-SNA is somewhat infrequent (and involves a regional jet). Like United American is expensive for last minute purchases/changes and charges a $100 change fee.

- Frontier (SFO-LAX). Many people like Frontier's in flight product and the prices seem to be lower, at least for last-minute travel. One big downside is a frequent flyer program with few international partners.

- Alaska (SFO-LAX). Alaska is a fantastic airline customer service wise, and you'll likely spend most of your time in free upgrades to first class if you fly SFO-LAX every week. Alaska's change fee is $50 and that is waived for people who fly more than 50 segments a year (or 50,000 miles). Alaska also has a fantastic frequent flyer program. The big downside is limited flights (only one in the morning and one in the evening).

- Delta (SFO-LAX). Delta just entered this market and I'm not sure what the frequency will be. I know little about Delta's FF program, but I'm sure you can get lots of information over at the Delta forum.

- Southwest (starting service in the fall of this year). Southwest just announced a return to SFO, and it will likely fly SFO-LAX as well as SFO-SNA. Many business travelers in the Bay Area love this airline for trips to SoCal and Vegas simply because it has incredible frequency and complete flexibility on relatively inexpensive full-fare tickets. The frequent flyer program only allows for domestic awards, however, and it still remains to be seen if Southwest will be able to provide the level of service at SFO that it does at OAK and SJC.