I spent the first 18 years of my life living in Park City, Utah (20 minutes from the U) and am now in my 19th year as a Fresman at CU Boulder. I have no doubt in my mind that I made the right decision, but I'm slightly biased because I didn't want to end up in the same place as 50% of my high school. If you are to come to Boulder, here are my suggestions.
*Join the Freestyle Ski Team. There's lots of cool people on it and if you ever looking to go up whether it be a week day or weekend, you can catch a ride with some good park skiers at that.
*Try to bring a car. Boulder is fun, but it's nice to get to Denver every once in a while and getting to the mountains is 10000x easier with your own car (plus, people pay for your gas if you drive rather that you paying for people's gas)
*Live in a general dorm. Don't make the mistake I did and go to some RAP or engineering dorm, live in a general dorm (Willard, Hallett, Libby, Sewell...). You'll meet a lot more normal people that are looking to have fun, and if you're not dumb, you can avoid the (previously stated) copious amounts of coke heads. Plus, just an opinion on my part (however, shared with many of my friends from the U of U), the girls here are much, much hotter. Just gotta do your best to avoid some of the dumb sorority ones (not a hard task at all).
*Be prepared to have a great time. Boulder is a fun ass place whether your among the thousands outside on campus on a friday afternoon enjoying the sunny weather or going out at night, there's never a stop in good times.
The U is obviously a great second choice (I've spent plenty of time hanging with my friends that go there). The partying is a little harder to come by (probably an under exaggeration, it's just not quite like Boulder), but it comes with a great silver lining in that the skiing is much closer, slightly less crowded, and there are more mountains. I'm not going to lie however, even after skiing Park City for 18 years, Breck park immediately has become my favorite park out there and I would rather put in the extra hour of driving as its just that much fun. Plus the Epic Pass is super cheap and gives you access to all the mountains you could ever want to ski.
Hope this helped, I went through the same dilemma last year myself and know I made the right choice after all.