+1
Also, look at what you can get later for doing this trip.
Better Seats
On UA, you get economy plus for all elites.
On AA you can get exit rows.
Luggage
UA- *A allows 2 bags of 70 lbs each all over the world for free
AA OW allows 2 bags of 50 lbs each
Domestic Upgrades
UA - possible, but less likely as a 2P
AA - possible, but you have to buy 30$ stickers / 500 miles flown.
UA so far appears better. But, AA - this is where it can get interesting.
Take the AA platinum challenge and qualify - fly 20k miles (10k miles with higher fare classes) after July 1 (like now) and you are Plat till Feb 2013. This Plat status gets you into all OW biz class lounges with a OW ticket (either award or paid)
I think AA if you can do a challenge. YMMV
This post is excellent advice. To clarify on the AA Plat = One World Sapphire = Lounge Access angle, you get lounge access on all international tickets, regardless of class of service flown. (i.e. even though you may be in coach. + the 100% bonus on RDM is nice.