AA wins:
- Higher domestic upgrade success rate because AA B738s and AA MD80s have a higher ratio of F than UA A319s and UA A320s.
- More generous international upgrade program. AA gives 8 transferable international upgrade certificates valid on any fare, UA gives less and they are not valid on the lowest fares.
- Better food in domestic First (only by a small margin, neither airline's food is great)
- When flying international, AA EXP gets you into Oneworld First Class lounges (BA, CX, JL, and QF have some really great F lounges). UA 1K only gets into Star Alliance business class lounges.
- Better phone support. UA 1K line is good but less empowered and less able to complete a wide variety of tasks themselves compared to AA's EXP desk.
- From Boston: AA flies to LHR (all year) and CDG (seasonal) non-stop. UA has 0 nonstop transatlantic flights from BOS. There are Star Alliance transatlantic on LH and LX but upgrade options are limited.
- From Boston: AA is partnered with Jetblue, they have the most non stop routes from BOS. AA's partnership with Jetblue is likely to grow closer (merger rumors, my prediction is an actual merger is unlikely but much more code sharing and reciprocal benefits to come).
- From Boston: AA partner JAL has a nonstop to Asia, Star alliance lacks any Asia non-stops.

UA wins:
- Larger route network, providing more domestic options from BOS. Star Alliance has more options than Oneworld in general.