I'll add my two cents: We've skiied in both Utah (Deer Valley twice, Park City once), Colorado (Beaver Creek, Steamboat, Snowmass, Keystone) and Montana (Big Sky). (Also Sugar and Beech Mountains in North Carolina but I have tried to forget that). For your criteria, I would heartily recommend both Park City and Steamboat.

I really liked Deer Valley with children, as it had an excellent ski school and NO snowboarders, and limits the number of people on the slopes each day so lines aren't long, but you do need a car.

While we have had altitude issues in Colorado (Keystone) we never had those issues in Utah. Flights into and out of Salt Lake City were easy, and getting to Park City is quick.

For our trip to Snowmass, I booked a more expensive flight directly in and out of nearby Aspen, pleased that I could bypass the commute to and from Denver. Unfortunately, we discovered as we waited at the airport to leave at the end of the trip, that airport frequently closes due to weather issues, and I had to scramble to get transportation to Denver and then buy a plane ticket on another airline to get home, hours later than I had originally planned. I will not fly into those small, regional airports again except maybe in summer.

As an added bonus during a Utah ski trip, one night my family went in to Salt Lake City and heard a rehearsal of the Mormon Tabernacle Choir. Not your usual ski-trip activity, and memorable!

Let us know where you decide to go.

Utah can be extremely cold. I prefer Tahoe or Mammoth.