How is the campus?
both nice. personally I think CU is a bit nicer, but again, both really nice.
Boulder is a lot smaller than Seattle, so if you are looking for a more big city vibe, seattle is where to be. Boulder has the hill and downtown which do kick ass, but again, city is not huge. Traffic is Boulder will be a ton better.
How is the "vibe" (a lot of snotty kids or a lot of chill kids etc...)?
You are going to have snotty little shits and chill kids at both. Both are going to have a rich kid out of state element, but you just need to do your best to avoid it. Both have awesome people.
How is the dorm life/parties/bars?
Again, both kill it. Dorms at CU are better, just because there are more dorms available. Multiple people I know at UW have ended up with 3 people in a 2 person dorm. not fun. Other than that, both dorms will be crazy and fun, as will the parties.
Boulder has a ton of bars downtown and on the hill, but as I said above, it is sort of limited besides that. Nice thing is you can basically walk to everything in Boulder without issue.
The U-district has a lot of bars, but walking to other sections of the city can be a pain, so taking a taxi or such might be needed. BUT there is Capital HIll, Fremont, Ballard, Belltown, and pioneer square which all have great nightlife.
How is the skiing?
Park? Colorado kills washington, plain and simple. Snoqualmie and stevens do represent ok, but you cannot compare them to keystone and breck.
Terrain? Personally washington kills inbounds skiing in Colorado. I grew up in Boulder and went to school in washington, if that says anything.
Pow? Lighter in Colorado, deeper and more of it in Washington.
Weather? Never sunny in WA. I have had less that 5 sunny pow days since being in the PNW (5 years). CO actually gets sunny pow days.
Groomers? If this matters....WA groomers blow. haha two things i miss about colorado are sunny pow days and groomers.
Ski passes are cheaper in colorado too, even with the student passes. Stevens and snoqualmie are affordable, but crystal is not. You can get passes to almost every colorado area for like $1000-1200.
Hope that helps.
Both schools kill it, have good parties, have hot girls, have great skiing, and are in great cities. Comparable academics and college is all about what you put in.
Basically you should visit and just decide if Seattle or Boulder is more fitting for you.