Which to take depends on what you want, miles or quality. Personally I prefer BA and there is a better chance of getting an exit row on BA due to the restrictions in pre-seat selection and the lounges are nicer and I find BA service and in flight entertainment to be better

If wanting to earn AA miles however there is a drawback. If you fly on BA from the USA to Europe, you will earn zero miles since AA restricts mileage earning on BA such that nothing is earned on those routes

You can fly from Canada to Uk and earn miles , though be aware if booked in less than H class you will earn only 25% miles

If tall, I would suggest checking seat maps and if 1 has an exit row seat available and the other doesnt , I would take that regardless of miles earned.

Going on AA or BA will have the drawback of having to connect en-route rather than got non-stop which would be a boig negative to me.