Get the ones with a hard plastic shell. Another thing that will save your knees from stumps in the coming years, once you're going intothe back country, and arthritis when you're a few years older will be to get into the habit of having a higher stance in your tele turn. Don't let your the toe of your back foot get more than about a foot behind the heel of your front foot. Most of the old duffers I know ski with a higher stance and know better than to go out without kneepads.
Kneepads also really help keep your knees warm (and prevent stiffness)when downhilling and are really handy to have for taking a break on your knees when you're snowboarding.
--Andy