Best mountain in Utah?

If you want steeps, cliffs, chutes, and gnarly gnar, Snowbird and Alta. Especially Snowbird. If you want snow, still Alta/Snowbird, but Alta gets more than Snowbird. Both get more than other places (i.e. PC). If you want a quiet place that doesn't get skied out as much, Solitude. If you want park... no idea.

best terrain & snow: snowbird/alta

I love bird. So much fun. Hoping to get a pass there next season. For park, PC or brighton. Brighton's got better terrain for sure, though. But PC's got a few spots that are real fun when it snows.

If you can afford duel passes that's the way to go. PC + Snowbird/Alta/Soli. If you can only do one I'd say brighton cause its the best all-around.

Snowbird is way better than alta

Hey dude, I was in the same situation as you last year! I'm going to be a sophomore at Westmini this year, you're definitely going to dig it.
As for what mountain to ride at, that will most likely have a lot to do with where your group of friends will ride but you should know what you want as far as the type of terrain you're looking for. I grew up in MN lapping rails on towropes and after riding the bird last season I'm definitely not getting a park pass any season soon.
Like some other people said Alta/Snowbird is where you want to be for hucking cliffs and chowin pow. Snowbird has a ridiculous amount of cliffs that you can hit at pretty much any point in the season. Snowbird is bigger, but also gets tracked out faster than Alta because of the Tram and allowing both skiers and snowboarders. Alta doesn't have as many cliffs as Snowbird (most places don't), but theres still a ton of them and theres always jumps built at Alta. In my season's experience I found that the ski patrol tears down most of the jumps built at snowbird while they leave them up at Alta.
Both are sick mountains and, honestly, you're going to have the most fun you've ever had skiing next season, regardless of where you ride.

I'm going to a Utah ski holiday in January

I'm from Australia and in North America have skied aspen, whistler and revelstoke before so I thought it was Utah's turn

We have two weeks. I'm thinking 9 days at Snowbird and 4 or 5 at Brighton. How does that sound?

Due to the distance we are coming we want to pack as much skiing time in as possible, so convenience is key. I think both are pretty close to SLC which should just be an easy connecting flight through LAX like Aspen was.

How does my plan sound?

I will be with snowboarders so that rules Alta out unfortunately. We would also consider Jackson Hole across the border but perhaps that can wait til a purely boys trip next time.

Deer Valley. Really though, it's the best.