I'd like to know this as well. Based on the guide, I'm between a M & L, and I wear a medium in the Ex-Officio and they fit well, whereas a medium in Smartwool's is a bit snug.
Edit: Either the sizing guide for these is strange, or my body proportions are more odd than I thought (probably a bit of both). Based on the chart, by waist size I'm halfway between a medium and large, by hips I'm a small, and by inseam XS. While this is the best deal by far that I've been able to find for these, with the singular exception of large oasis/night (blue) ones at campsaver/opticsplanet, I may have to pass if I can't figure out what the right size is. I'm leaning toward medium, but I'd hate to buy some only to have them not fit, as I'm assuming there's no return/replace option.