My comments are from my NA summer spent down there in '95. "traditionally" Termas gets a lot of snow. Its farther south so just gets more storms than do the areas around Santiago. Its the equivalent of Washington vs Ca. WA traditionally gets more consistent snow, but when it snows in CA it means 3-5 feet. Same thing in Chile, but Valle Nevado/Portillo/LL are like CA and Termas is like WA. It is lower in elevation so the snow is wetter. The off-piste and exploring around there is excellent. That is really the main reason for going down there, the touring off the top is excellent. Bring pieps/shovel etc. It is a bit of a haul from Santiago, but you can get into some more traditional friendlier Chilean towns. They have one of the longest slowest double chairs on the planet. I think it took something like 20 mins. Bring music or someone to talk to as I was solo and almost fell asleep each lift ride.
Las Lenas is the Sh*t! Best terrain down there, period! They get the same weather as the Santiago areas more or less, so if its dry at Valle Nevado it will be dry in LL as well. I personally didn't think Portillo was that great. Not particularly big, but it is beautifull situated on the lake. I'm thinking of heading back down there for the first time since '95, and I'm only considering LL. What else you want to know? Food is better in Argentina and now it should be alot cheaper than when I was down there.