I think the lack of responses is due to the fact that you don't (a) indicate where you'd be going to use them, and/or (b) you don't indicate what future plans you might have where either source could be used.

I for one, and I expect many of us, do an informal (or maybe formal) plan for burning miles that extends over time, usually past the next trip.

In that scenario, you might be better conserving the AA miles for a trip where BA fuel fines would be an issue otherwise (e.g. using BA transatlantic) or where an AA Oneworld award (arguably one of the most powerful FF redemption vehicles) would be the chosen approach.

Or you might conserve the BA miles for, e.g., a N. America < > S. America award, where the mileage requirement is lower than AA's for business class travel. And so on.

Personally, with ATI looming and the likely integration of many of BA's, IB's and AA's operations over the coming year, I'm building my AA balance this year so that once the FF plans are "aligned" (whatever the hell that means - be afraid) I'll be able either (a) burn them in a hurry if the program sucks; or (b) congratulate myself on being grandfathered into an even better system.