I know most people are suggesting AS, but one benefit of the AA program is that you can book one way award tickets on AA partners as well as on AA metal (which you can't do on AS). Furthermore, on AA partner award tickets you can mix-and-match partners, so you could fly AS-BA-IB-AA if you wanted to. That can really give you a lot of flexibility when you're trying to find an award ticket to Europe during peak travel periods.

While having Delta as an additional domestic partner might seem like a plus, keep in mind that it's awfully difficult to redeem with them (this isn't exclusive to AS, DL's own frequent fliers have a hard time using the cheap award buckets too).

Ultimately they're both good programs, I switched from AA to AS because AA got rid of the PDX-ORD non-stop that I fly with some frequency, but I'd have no qualms with switching back.