Really not much difference, to be honest.

Haven't flown BA Y much of late, so my observations may not be current, but it seems to me they have even less Y legroom than AA.

The AA 772 has seatback TVs and a decent IFE system, plus the Y seats are more comfortable than the old AA ones.

The big question is whether the 772 on the route you are flying has the MRTC (more room through coach) reconfiguration, where AA have taken rows of coach seats out to give more leg room.....why not ask the guys at the AA board on FT if any of them fly that route and would know ?