I've been to both mountains for one day last February.
Sundance was one of the nicest places I ever sked. Absolutely no crowds (I think it was Monday), beautiful scenery, weather couldn't be better, blue skies. Being my first time there, I've explored the mountain which was good enough fun. Base area is minimalistic, locker system pretty unique, least environmental impact you can imagine at the ski area.
Snowbasin on the other hand was way too crowded (ok, it was Saturday President's day weekend), drive up was tedious since the weather was all fog and blowing snow, couldn't see thing on the mountain. Obviously the mountain has great terrain, but it just did not work for me that day. I don't think I've ever seen so many people in the base lodge or in the restrooms or in the line for food. Children were sleeping or playing on the floor all over the place, stuff scattered everywhere, it was just not cool. I was impressed by the fancy base lodge, heated walkways and the condiments assortment at the caffeteria (
). I hope next time I'll get no crowds and nicer weather.
My advice would be, if you can, go to both places. I "sacrificed" two days of Cottonwoods last year to see 2 new hills and it was worrth it.