What will he be using his card for? That probably decides which programme is the best.

I think he is not looking for award travel. (For award travel, AA is the best, but he cannot use it for non-AA upgrades. If he is flying CX, AA miles cannot be used for upgrades.)

If he wants lounge access:
* AA is the worst. He cannot gets into an OW lounge when traveling domestically.
* BA will put him in the EM category, but if he flies between US and UK, he cannot get BA miles & tier points on AA flights.
* If he can eventually end up in the CX DM tier, then he should go for CX. He has more flexibility when he flies CX, and even when he is flying CX.