I would go thru jfk, the LHR layover time is really short on the DFW flight, especially if you have luggage you won’t be able to check it thru on separate tickets, on aa equipment you will have to change terminals in Heathrow and Heathrow has very ridgid cutoff times, the First on AA equipment is pretty bad these days so I am not sure I would opt to spend more time on the plane, I don’t know where you are in the dc area but IAD can be a pain to get to compared to DCA,