Stwildcat,
I have been to Chile to ski. I have not been to Portillio but the conditions at Valle Nevado are similar. The storms in Chile tend to be big, they can last for sevaral days and they usually come with 70-100 mph winds. As well, most of the skiiing in Chile is above the tree line which makes skiing in white out conditions quite difficult. Resorts often close lifts in the strong winds. After the storm, you can get some amazing powder days, and in general the number of skiers at the resorts is quite modest vs. what we are used to in the US. You can ski fresh lines for as long as a week after a big storm.
The real problem with Chile is what I mentioned before. They don't get a lot of storms. The snow comes in big bunches, and it can be weeks in between storms. Having said this, you definitely want a ski that you enjoy on hard groomed conditions, because if it doesn't snow, this is what you will be skiing on. An all mountain ski that does well on groomed terrain and that will float in some pow (Head IM 82's) would be a good choice. The other option would be to call Portillo, and find out if they have powder skis to rent. If so, I would bring carving skis and if you get lucky and have some powder days, then you can rent fat powder skis.
I would mentally plan for hard snow conditions (becaiuse this is what they have for a good percentage of the season). If this is what you end op getting you won't be disappointed. If you do get some fresh snow you will be even more excited. Think of it as similar to carrying an umbrella on a day that it might rain. When it doesn't rain you are pumped, but you are prepared no matter what.