I was looking at the same issue for my upcoming RDU-NRT-SIN flights. I went with DFW as it was where the upgrade was available. As much as I hate paying for a service that should be free at aa.com, I found ExpertFlyer to be very helpful in making the decision. Personally I like the longer international flights so I can sleep, eat, and watch a movie or two.

Two flights from DFW so that would be my choice if upgrades weren't available in any of the cities.