One other consideration might be which US-based FF program you belong to, if any. If UA, you don't get the same bonus mileage benefits flying Star Alliance partner SQ in C or F as you would with UA itself. If AA, you do get those same bonuses on One World partner CX as you would with an AA flight. Also, if you're EXP in AA (or top status on other OW airlines), you get access to Cx and other OW F lounges regardless of which class you fly. Not so with UA re its partners.