wich resort should l pick im an advanced skier l will be 14 or 15 when l go also l like park skiing and l live on the east cost so a plac ethat will give me good snow conditions mabey even a pow day

Either resort can give you great powder. The Whistler/Blackcomb resort is one of the best in the world with outstanding parks and slopes. The Alta area has a reputation for great powder.

It really depends on what u like. If u r like me, & u like sick lines & parks, than l would definetley go & check out Whistler/Blackcomb.
Now ifyou r more of a huge pow person, l would go to Alta or Snowbird. There r definetley some ups & downs to both. Whistler has a lot more vertical & is way bigger, yet the snow is usually more wet as they intercept a lot of storms off the pacific coast.
Alta or Snowbird is snow is a lot drier & fluffier, but they definetley do not have awesome parks & more challenging terrain.
You r going to get a ton of pow days at either place. This is because Alta/Snowbird gets lake effect, & whistler has the storms coming in having picked up tons of pacific moisture (larger lake effect). Both resorts benefit from the orographic effect, also.
Based on ur situation, l would definetley go out to Whistler. You will be able to ride lots of park, ski some nice snow & more challenging stuff. Have fun!

Hope this helps!

l would say wistler.
the parks are great.
and the glades are awesome.