Cities and Knights of Catan: Which Field First?

[VoxRationis]

So, for those of you who've played Cities and Knights of Catan, which of the three fields (trade, politics, or science) do you prioritize when you set up your first city? Each field has its own advantages (though I doubt very much they're all equal). I myself favor Science, and I'm not sure whether that's because of my personal bias towards science as a real-life field, or because of the Aqueduct power, or because of the usefulness of wood.