Originally Posted by JJeffrey

From a connecting experience perspective there's virtually zero difference in connecting IB>IB at MAD versus AA>IB.

At MAD, your longhaul flight from the US will arrive into T4S (both IB and AA), you'll clear immigration then take the train to T4 where your AGP flight will depart from.

And in the US, there shouldn't be any problem with a AA>IB luggage transfer. At any of the IB gateways (JFK, MIA, ORD, DFW, etc.) there will be hundreds to thousands of bags that get transferred from AA to IB (and other partners) on any given day. The baggage handlers will be well versed in this.

All else being equal I'd take whatver schedule works the best.