Both programs have strengths and weaknesses and it is impossible to say which one is best overall.

Using UA miles one can fly from Asia to South America and have a stop over in US for 150K in F i.e. HKG-SFOx-LAX-DENx-YYZx-GRU-ORDx-NRTx-ICNx-HKG or for the same mileage you can fly from US to to Japan in F and have a stop over in Europe.

You absolutely CANNOT (99% or more of the time) change the UA issued *A award once the travel has begun where AA allows date changes.

AA's OW mileage based awards are truly nice i.e. RTW LAX-LIM-SCL-EZE-GRU-MADx-JNB-MAD-TLV-LHR-LCA-LHRx-HKG-NRT-MEL-HKGx-LAX for 220K AA miles in C.

If you reach certain threshold on UA you can earn quadruple miles for a month that makes MR's so much more "enjoyable".