Rather than comparing AA to BA, better to list out what is important to you, including of course, cost & schedule

1. Seat (width & pitch).
2. Selection and when selected.
3. Food
4. Liquor
5. Entertainment / WiFi

AA makes no bones about the fact that it puts its better aircraft on more profitable routes. You can expect a better experience JFK-LHR than PHL-GLA. Whether that makes it worth flying to GLA with a connection at LON is a matter or personal preference.

For most, this all begins and ends with cost & convenience.