Lounge options on both ends of the flight are much better if on the AA flights. As oneworld emerald OP would have access to:

• AA's Flagship Lounge at ORD T3 (instead of the terrible BA lounges in ORD T5)
• If OWE through AA or BA access to AA Arrivals lounge at LHR (which is better than BA's arrival lounge at LHR T5)
• If OP also has a return LHR-ORD flight - the lounge options at LHR T3 where AA's flights depart from are far better than BA's lounges at LHR T5.

UK261 protection ORD-LHR is the only reason I'd consider taking BA metal on this flight.