If you plan enough credit card spending, keeping the UA card gives you *A flight options when it comes time for award travel. Earn AA miles by flying AA, and you'd have access to oneworld flights. Plan your award travel by taking *A in one direction and oneworld in the other. Since AA does not permit stopovers on roundtrip awards, you are basically giving up that perk by leaving UA anyway.
YMMV, and good luck!