For oneworld, if you occasionally will be flying AA transatlantic and other times flying BA, neither of their programs is going to work out for you all that well. You'll have to dilute the miles regardless of which one you fly.

Assuming you don't care about getting upgrades, just about making all your RDMs qualify, you might even consider choosing to join one of the other OW programs such as LAN (LA) or Qantas (QF). Then your transatlantics would count to the same program regardless of whether you flew AA or BA. Or even a non-OW program like Alaska Air (AS), where both AA and BA earn miles. AS has a number of partners including some airlines not in any alliance and some Skyteam airlines like KL, DL, CO, NW. Not all of their partners earn status EQMs (BA does not, AA does) but it would be a single bucket for all your non-*A flying.