Cuddle Duds is a brand of long underwear that is a cotton/nylon combination - with the cotton next to your skin and the nylon on the outside for smoothness. My mother in law who is always freezing cold swears by them. I think they are thicker than the silk. I have always worn lightweight silk for travel and synthetic (such as Patagonia capilene) for hiking.

I have both lightweight and medium weight from Winter Silks, and for really cold weather I like the medium. They are really still very light weight, are not bulky at all, take up little room in suitcase, etc.

Silk for me.