Each program has its own Pros and Cons, only you can really decide which is more appropriate for you. There is no straightforward answer. If you are able to book Premium Economy fares then BAEC may be better for the ability to upgrade to CW. Else, AA may be better with the System Wide Upgrades (although you'd need to fly AA metal TATL).
Thanks. Think will continue with BA. I can't stand AA's economy (if I'd have to stick with AA TATL).
The other good thing is the household account with BA (I presume there is no equivalent with AA?) which enables sharing avios easily. (Though I guess you can just use AA miles to purchase a ticket for someone else?)
However I am always amazed how much in taxes you have to pay when you purchase BA tickets with miles (something that's supposed to be 'free'). That and the availability for reward flights (non-existant with BA). I presume this is where AA really shines.