I would add that the 77W is guaranteed on the AA50/51 rotation. From the AA51/LHR-DFW perspective, it is actually better from an immigration standpoint in that you get to DFW before all of the other TATL AA and BA flights (BA193 tends to land right in the middle of the big immigration mess). As a Global Entry member, I don't really care for myself, but I am certainly sympathetic to those that have to wait hours in visitor immigration queues.

As long as you're not trying to MFU or redeem a 2-4-1 in J, I would say give the AA flight a chance.