A factor arguing in favor of BA is that BA silver gives you access to all U.S. Admirals Clubs. AA Platinum, the equivalent, gives you access only when you're on an international itinerary. Since most of your travel is domestic, that's worth something: no more than an AC membership, perhaps less.

In the other direction, it's a lot easier to achieve status with AA via discount economy class travel. BA has as much as a 6:1 ratio of tier point earning between discount economy and first class. AA has at most 3:1; and depending on how you qualify for elite status, there may be no difference at all.

Finally, if you're after AA upgrades, you want to be an AA frequent flyer, period. Nothing else will do you much good in that regard.

My recommendation would be to go with AA and pay for an AC membership if it's worth it to you. If you can, make the switch at a point in the calendar year where you can earn AA status in what's left of it - AA goes by calendar years for everyone - and retain BA status until your AA status kicks in.