The only difference in Y between the 2 airlines when crossing the pond is the booze charge. BA =0 AA =$4-5 per. THAT's IT! That's assuming you fly on AA 777, which you will do when flying from MIA to LHR.

If somebody tells you otherwise, they are full of it.

Now you will have to make the decision whether you want to earn 25% of miles that also require more to fly for free or 100% on about 95% of the cheap fares codes, (Tickets between North America and Europe, India, Asia and Latin America booked in O and Transatlantic tickets booked in Q are not eligible for mileage credit.) and are easier and in many cases require less to use.