The meal quality on AA is better than BA in PE. I flew out to the US 2 weeks ago with AA and then back with BA. The AA meal feels more like a business meal where as the BA meal feels like economy. Also the 2nd BA meal is pretty much 1/2 a sandwich whereas you get something more substantial with AA.

As others have said, on the return you won't be covered by UK261 if you fly AA. I tend to always book AA on the outbound (as the lounges in T3 at LHR are better) and then BA home (especially since BA have moved to T3 in Chicago and has a better lounge).