I regularly drive into the mountains of WV to ski. These are some crazy mountain roads that are not cleared very well. It used to be a scary experience in FWD cars, even with snow tires, but my last three cars have been Outbacks and they are great in the snow. Now, on the other hand, if YOU don't have a solid appreciation/need for AWD in your ski driving, then maybe it's not required where you drive! When I lived in New England, I got by fine with FWD for many years. I guess to me, the bottom line is that you would not be considering FWD unless it was feasible for your driving, right??
Keep in mind that the Prius is a small car platform with a very limited payload and gross weight rating. Make sure you won't exceed it's ratings when you load it with people/gear. A lot of people buy Honda CR-Vs and Elements, and don't realize it's based on a Civic platform and has a limited load capability. It's just not safe to load these vehicles up past their recommended rating. Among other things, it seriously exceeds the tire's load rating.
Everything I have heard about the Pruis has been great and I am a big fan of the car and technology. Not sure it would make the best ski vehicle though. Have you considered the other hybrids that are better suited to active lifestyles?? Ford Escape hybrid, Lexus RX400H, Toyota Highlander Hybrid are a few that I can think of.
BTW, the Prius has a low drag "Kamm-back" design on the roof and rear end, and I am fairly certain any sort of roof rack or roof box will really increase the drag. Same goes for snow tires -- they will be considerably stickier than the low-rolling-resistance tires the Prius comes with. So on those two fronts, you'll be hobbling the capabilities of the car. And driving in the mountains, you will likely limit the hybrid powertrain's effectiveness. Look at some past articles about hybrids by Pat Bedard in Car & Driver. He is a fan of hybrids, but has run into several cases where they did not live up to their mileage claims. Mountain driving is one scenario.
P.S. My 05 Outback turbo requires premium fuel and gets 22mpg on the highway, a lot worse than my old Outback (which hit 28-30 on some trips). So I am spending a lot more on gas than before. But, the car is wicked fast (250HP/250FTLB) and a blast to drive. For me, the added fuel expenses are a fun tax I am willing to pay. Maybe someday I'll want to get better MPG, but for now I am enjoying the performance.