The term "snow pants" covers a really broad range of stuff. Everthing from heavily insulated pants, to shell garments. What works best really depends on what you will be doing.
For emergency gear to carry in your vehicle, insulated pants that go over one's regular street clothes might be a good choice. Another option would be insulated coveralls, such as Carhartts. This sort of gear would be great to protect you while you dig your vehicle out, or put chains on, or wait for rescue. But it might be rather constricting and maybe too warm for active hiking or skiing.
For back country skiing I prefer shell type pants, made of gore tex or similar type wind and water proof fabric. I wear these over smart wool or polypro long underwear. I use the side zips to ventilate if I'm getting too warm. If it is really cold I substitute lightweight fleece (instead of long underwear) underneath the shell. This set up is very versatile for a wide range of temperature. It is also not too binding or constricting for skiing.
Whachever choice you make, it is important to size them to easily fit over any layers you might wear underneath. Remember, that each layer you add makes you bigger for the next layer on top! Before paying for expensive gear, it is always a good idea to try it on in the shop, over whatever layers you might wear underneath. My shell gear (jacket and pants) is all quite big, to give me plenty of room for layering. I'm not very stylish, but I'm comfortable in a wide range of conditions.