What type of bicycle would you choose for a cross country trip?

I have a Trek 2.1 road bike and a Gary Fisher Hoo Koo E Koo mountain bike. I am planning on cycling cross country from the coast of South Carolina to Northern California. For this trip, which of these bicycles would be preferable, or should I go with something different, like a hybrid? Also, for my gear, which utility / storage trailer is good to have in tow.