Yep...flying on BA with AA plat gives no extra luggage allowance. Flying on AA with BA silver, you get the full (albeit less generous) elite luggage allowance.

If you are planning on flying a lot of US domestic in economy, AA is the obvious choice (allows upgrade instruments). If you aren't planning on any WT+/J bookings, then AA will again be easier to climb of the status ladder. Otherwise, BA status can achieved for far, far fewer flown miles, if one books into higher cabins.

tb