Very cool!

My vote goes for the black shirt with orange/white combination. The "X X X" in grey looks a bit depressing, as opposed to the vivid (and original) white colour.

Then... if we would have still budget we could do like the cool FLOSS conference do nowadays: a special shirt for the volunteers in the organization. A combination of usefulness (easy to spot who can help you) and gratituted (a Thank You! shirt) ... If that would happen I would go for the orange shirt with white fonts.