I've been flying that route with AA and BA for 25 years and have been very happy with them. Irrespective of whether you choose to credit your miles to AA or BA (or both), you always can fly either of them and credit the miles to whichever program works for you. I tend to prefer flying BA (especially now that they use their new business class suites on this route) and crediting my miles to AA, but I've also have flown AA countless times without an issue. If lounges are a big deal for you, you might want to fly AA rather than BA out of LHR. Although you'll have access to the same lounges out of JFK, at LHR flying AA gives you access to the Terminal 3 lounges (including the CX lounge, which many consider to be the best), whereas flying BA will give you access only to the BA lounges (which are fine, but most would say not nearly as good as the CX lounge in Terminal 3).