Having done both on AA and BA,
To ORD I would stay with AA, all aisle access is wonderful.. (dont like climbing over others)
From MIA ,well AA on that one too,
But saying that I have enjoyed BA on the upper deck, its quite.. ratio of FA and bathrooms per passenger is wonderful
just off topic.. having done BA J out of ATL, I swear BA must find the oldest 777 in their inventory, and use it on that route, both form LHR to ATL and ATL-LHR