I think you should take a long term view - where do you see yourself in a few years?

I also think its best to belong to the scheme of the airline you do the majority of your travels with. AA is pretty good at giving BA emerald most of the benefits, but it isn't quite the same.

I think the basic tradeoff is:

BA for lounge access in the US and potentially building towards LTG

AA for domestic upgrades and cheaper mileage redemptions

Also AA wont status match, so you will need to build status up with them, if you think AA is a better bet.

PS welcome to the board