Originally Posted by Thunderroad

For the above reasons, and a few more, I'd suggest AA over UA. (Can't really comment on DL beyond its reputation for having a lousy FF program.)

As already noted, UA's FF program has deteriorated over the past two or three years and shows every sign of continuing to do so. AA's is considerably better. In addition:

1. Flying out of SFO, greater likelihood of upgrades as an EXP on AA than as a 1K on UA, since there are so many more 1K's based here and it's a second class status to GS. In contrast, EXP is the highest status on AA. If you achieve GS and/or will fly very expensive economy fares on UA (since UA prioritizes upgrades within a status level based on ticket price, whereas AA does so based on date of purchase) that calculation may change somewhat. But you still might come out ahead on AA or at least fare as well.
2. EXP customer service line considerably better and more consistent than UA 1K line.
3. True that Star Alliance offers more options to Europe, but AA One World partner BA has a better business class than UA or its partner LH.
4. If for a given trip J pricing is beyond your company's budget, BA at least has the intermediate cabin of World Traveler Plus so you would not have to drop down to pure economy.
5. As an EXP, you get access to some very nice first class lounges of AA partners overseas, including BA's excellent LHR F lounge, regardless of what cabin you fly in. You don't get access to Star Alliance F lounges unless you actually fly F.
6. Should you cash in miles to vacation in Asia, AA partner CX is better in F and (as it introduces new seats) in J than UA partners. The exception is UA partner SQ, but redemption on SQ is very difficult.