Originally Posted by
guv1976
I'll just note that since AA and BA sell each other's flights on this route (usually as codeshares), it is possible to fly one carrier on the outbound and the other carrier on the return, while still booking the roundtrip in a single transaction at either aa.com or ba.com. So perhaps AA metal on the outbound (to take advantage of the T3 lounges), and BA metal on the return (for the UK 261 protections).
Have done exactly this on a couple of occasions for these reasons (plus the fact an INT>DOM connection is much easier within T5 after a night flight).
OP, this could be a good option for you as it gives you first-hand experience of both to compare.