I will also add that while the poly comment about the fire was meant to be mildly amusing, I prefer wool for other reasons.
As far as brand, I have had good results with smartwool. And their socks are great too.
One big reason is that it seems to me the poly stink sooner. Many times I am using these types of underwear it is because I am camping and hiking, and so don't have easy access to a laundry.
Because lets face it, I am not wearing long underwear on a regular basis in Texas, its usually for an outdoor adventure of some duration.
I cannot say much about polypro long underwear, but I can say that I do use UnderArmor extensively as part of my workout clothes (and wash them often) and have had great results.