Regarding AA, i prefer booking transatlantic flights with American because you can book seats when you make the reservation up to 11 months in advance. My next flight is with BA in a few weeks and i had to pay an extra $76 to ensure i get an aisle seat and not stuck in the middle of the row, this i dont mind so much but i wish BA would let you book exit and bulkhead seats at time of reservation especially as they are charging.
Seat selection is free 24 hours before departure and if I make that roughly on time I always get my seat if choice. Doesn't gaurantee a family will sit together but travelling on business alone, it works fine every time to avoid the naff seats.