Having just moved to the area, and never skied either state before, my wife and I just went through the process of deciding where to go for a week skiing before Xmas. From Phoenix, SLC and the interior of CO are both about a 12 hour drive.
Based on current snowfall, open terrain, hotel cost, location, ease of access, ease of booking; there was one clear winner; we booked a five night/four day trip to the Cottonwood Canyon resorts in SLC.
For just under $900 we get five nights in a mid quality chain hotel in SLC with breakfast. Included is a four day "Super Pass" to Alta/Snowbird/Solitude/Brighton. The pass also includes free access to the city run ski bus, so there is no need for a 4x4 rental, and access to downtown on the light rail. The price is for two and was all booked through one site, very easy. Plus you get a free sticker pack!
We could find nothing in CO that even came close to matching this deal, either the economics of it or the snow/terrain conditions. Our opinion is unbiased and just based on current conditions and deals.
And unless it's a once every 20 year thing the skiing in UT will be better without question...anyone who questions that is just being naive or a homer!