Well UA will have much more frequency ex-SFO, as well as several non-stops to Europe (LHR, CDG, FRA), whereas AA has none.
One thing to keep in mind though if you will be doing SFO-JFK frequently is that UA has effectively killed upgrades on that route. It's selling all the empty J seats at check-in for $200-$350, even with long lists of 1Ks and Plats waitlisted on instruments.
Also pretty difficult to use UA GPU's for TATL's from SFO. Asia is considerably easier.