I would go for US/AA i this situation, because of oneworld.
If you get paid premium economy, BA and CX have decent premium economy products and aren't that difficult to upgrade with miles on their respective programs. For the Europe to US daytime flights, how much value would you put on a Premium economy to Business upgrade. Sure on a red-eye it is very valuable, but less so on a daytime flight. UA E+, AA MCE and US Choice/Preferred seats are all standard E seats. Premium economy (with an emphasis on premium) gives you more width and is closer to domestic first.
BA lounge in PHL is very nice. BA lounge in EWR beats the UA Clubs in EWR. One world Emerald gives you First class lounges which can be much nicer than business lounges at outstations.
If JFK was a decent option, the argument could be quite different. BR (Eva), SQ (Singapore) from JFK provide decent options across both the Pacific and the Atlantic. Lufthansa, Swiss, Turkish also round out the offerings.
I'm a US CP - and the move from star to one world was a huge benefit for me. I enjoy flying BA/CX and AA quite a bit. I will miss BR, TG and SQ.
The other key question is where in Asia or Europe the OP is flying to...