LHR-JFK-LHR is my most frequent LH route. I will always try to take AA on LHR-JFK and BA on JFK-LHR.

The main reasons are as follows:

AA LHR-JFK:

• Better J product for a day flight - seat, IFE, food is more to my liking (although BA does have a better drinks selection)
• T3 lounges are better than T5
• With fewer flights out of LHR, I find AA more punctual

BA JFK-LHR:

• Ability to sleep from gear up (AA keeps their seat belt sign on for too long in the climb, and wakes up the cabin earlier to collect Bose headsets - BA you can sleep longer)
• Generally will try to take a 747 Upper Deck for a quieter, less dense experience, that helps to sleep the whole flight
• Easier connection onwards in the morning, within T5
• EC261 protection if things go wrong