We get into this every year! <g>
But here are pictures from a trip i took a couple years ago on 12-9. Snowbird was awesome with a 50" base.
Then another year, I was scheduled the same weekend, and was oh sooo close to canceling, I think the base was only 30", when it began snowing about 5' foot as i was flying out, the skiing turned out to be quite excellent.
For sure January is a better bet for everything open skiing, esp if you want the more advanced terrain. If you are looking for intermediate, family terrain then you likely have close to 90% chance of having plenty of terrain open for you.
I like December skiing because I was able to use my freq flyer miles easier and its' just a nice feeling to get a trip in before xmas at a quiet resort.
If this is your only trip of the year or you haven't done much big resort skiing, it's a better idea to wait to mid-Jan. That's when I'll be there.
If have to wait until then is going to stretch you somehow, then I would say you have a 75% of very good skiing on December 10th. At Snowbird/Alta that is. Too early for PC & Snowbasin.
Another option is to save money by staying in MidVale/Sandy in January, because the prices are same in Dec or Jan. It'll just take a little traveling each day.