I would think that it would depend on where you live, level of access to AA and UA flights, and where want to go?

I think both programs have there pluses and over all are about the same. I have better access to AA from my location but prefer UA (out of habit, I guess, and as a 1K MM on UA I already have the benefits).
Note: prior to United's PS service LAX-JFK, I felt AA had the better product on this route.

It's a matter of those factors mentioned above as well as pricing and personal preference.