I've been to both Keystone and Copper and will soon go to PCMR/Alta/ etc., so I'll have to reply again afte the trip but - I'd say right now Utah has an awful lot to live up to. Keystone and Copper are both excellent ski mountains. Keystone has 3 peaks + great night skiing, with diverse terrain, good lifts, easy access, and nearby towns and restaurants.
Copper is even better for skiing with fantastic bowls, great terrain for all levels of skiers, and a nice base area - if not particularly loaded with restaurants or apres ski options- still a comfortable number of options.
I'll be staying in Park City in a couple weeks and will let you know what I think, but right now, I'd say Utah, in general, has a long way to go to live up to the resorts I've been to in Colorado. Especially Copper Mountain. We shall see.