Both of them support Crossfire but the P55 chipset is not really designed for it. It works, but as you have seen, with two video cards, the speed of the 2nd slot will be limited because its bus is shared with other devices.
It's really up to you which of the two you prefer. Compare their specs and their slot layout (http://bit.ly/8hpZFg)
and decide based on that. Personally between the two I'd get the GA-P55M-UD2 but the answer depends entirely on your actual needs.