You can book AA or BA on a BA corporate TA deal presumably using BA codeshare. Take a couple of each and make your own mind up - maybe different one each way ? Outbound try a T3 vs a T5 depart, see the different lounges and of course the different on board. Sticking to OneWorld will get you Gold fairly quickly, which will make travelling on both rather nice !
I'd be inclined to go AA out BA back