Polypropylene, no question. There is an old saying about being in the outdoors when it' s cold, " COTTON KILLS." Search and rescue teams find dead people in the cold outdoors all of the time, they are usually wearing cotton. Don' t wear cotton in cold weather, period, not for ANY layer. It just holds moisture on your body. At least if poly' or wool gets wet, it still insulates to some extent.

PS- I know some snowboarders that don' t let the cold stop them, but their all about 17 years old, I don' t think that it' s their underwear keeping them going, probably their youth. When I was that age, I could have skied in my boxer shorts and not felt the cold.

Here is a trick to help keep your feet warm. Mine get cold even if I wear poly' socks and wool socks over that, then my boots. Problem is my feet sweat, then get cold. Here' s the trick, spray your clean bare feet with a spray anti-perspirent, especially the soles of your feet. It keeps your feet from sweating and they stay warmer. It really works, try it. I do it every day, even when going to work. (indoors) My socks are much dryer at the end of the day.