So I've been looking at purchasing a bivy sack to use with some sort of tarp/floorless shelter/snowcave set up. Because of this, my main concern in a bivy shelter is the breathability of the fabric and its ability vent moisture. I've looked at Black Diamond's nanoshield material, Integral Designs eVent, gore-tex, and many other kinds of fabric.

Does anyone have a clue what would be the best choice in fabric/model?