I like plastics for long, straightforward things, like going up 40-50° snow/ice all day.  Plastics are a very nice, comfortable platform for this, and yes, they are warm.  They are easier on your calves, and easier on your toes when you are kicking all day.  They also ski better if that is a factor in the equation.  I have found the Scarpa Inverno to do all these things quite well, and it has seen action around the globe for years now.  I have hiked in long distances in them, and to be honest, I think they might hike better than the la sportiva leathers I have. 
  
I'm sure someone would be willing to sell you some, as plastic boots are not the trendy thing right now.