Let me know if you end up getting those Mountain horse breeches! I've never gotten them because I can't imagine riding in something that looks and fits like snow pants.
My absolute favorite are Kerrits Sit tight and warms. They are BY FAAAAAR the best winter breeches I've ever owned! They're warmer, they hold up better, they fit better, they're more comfortable, I could go on and on. I would have SEVERAL pairs... if they had a pocket. I need my pocket. ANd sadly, they don't. My only complaint. They're a bit expensive but worth every single penny.
I also have a pair of Ariat Powerstretch Tights. I'm not impressed. They aren't that heavy or warm and for the same price I'd much much rather a pair of Kerrits.
I also have a pair of TuffRider Soft shell breeches. They're water proof and windproof but not very warm. I only wear them if it's a rainy day, no colder then 45 or 50.
I also have a pair of TuffRider Winter breeches. They suck. I hate them. They aren't very warm (I can feel the wind go through them). They say the smooth outter later repels hay and shavings? They pilled the first day I wore them. They're now so pilled all over that the second I come to hte barn I'm immediate covered with shavings that will NOT come off! The gripper things at the bottom cuffs are terrible too and very uncomfortable.