I have a pair of 5k pants and a pair of 10k pants. I honestly can't tell a difference between them. If you stay upright, both will keep you dry even on a pow day. If you spend a lot of time on your ass, your ass will get wet in either.
To hell with all that 5k, 10k, 15k, etc. stuff. If you see a lot of shit conditions and spend a lot of time flailing around, go out and get you some Gore-Tex gear, eVent, or Conduit (Mountain Hardware). That shit'll keep you dry regardless.