My opinion, coming from someone who lives in neither place, but has visited resorts in both states (Summit Co., SLC area, southern CO):
Powder- UtahPark / pipe- Colorado, although I'm not really qualified to comment too much on this stuff, since I rarely venture into the park or pipe.
Terrain- easily Utah, although I'll make an exception for Silverton, and Wolf Creek for the fun factor.
Distance from the hills- Utah. Still, you're gonna deal with heavy traffic in the Cottonwoods and on I-70.
I'll add that the public transportation option to the resorts in the SLC vicinity is a very good one, and I do utilize it. No rental car necessary, picks you up right downtown, and if you get an early start, you're still up on the hill pretty early.