I'm 1K on UA and EXP on AA. To me, their domestic products are similar -- UA is slightly nicer right now but AA should surpass them in 2014/2015. Upgrade percentage (I'm out of ORD) are also pretty close.
If you're in Manhattan or BK, I would strongly prefer LGA/JFK if I were you, which is heavy AA territory.
As far as applicability of miles, others are right in that Star miles are better for flexible use, but also consider that UA is moving to PQM next year, so you may have issues qualifying if you're a budget traveler.
Ultimately it's up to you though. They should be pretty close. Don't stress about it too much.