Technically you shouldn't be asking this question. If you need to decide where to ski and any of your options are in the west, you need not even consider the east.
That being said, I used to ski at Killington VT
. I went there for about 6 six seasons and learned to ski there. Then we decided to go out west, the rest they say is history. I've been skiing out west now for 7 seasons.
I've been to most of the places you've mentioned.
Copper is a great mountain. The mountain is naturally divided by skill level. They have a few great blue cruisers that run over 2500ft vertical.
I've stayed at Keystone twice. They seem a little more family oriented. You can share the Keystone ticket with Breck, and if you buy a certain amount of days they will give you day at Vail or Beaver Creek. I have skied Vail and wasn't impressed.
The good thing about staying at Breck, Keystone, or Copper is that they are all close to eachother.
Now if you want the greatest snow on earth, get your arse over to Utah. Alta and Snowbasin are excellent mountains for blue skiers.
And one last thing, when you are comparing trail maps, please remember to look at the acreage. Killington looks huge but is infact only 1200 acres. Then if you look at a Copper map, you'll see they something 2300 acres. My point is the mountains out west are much bigger.
Sometimes I go about in pity for myself and all the while a great wind carries me across the sky.