just because Ft. Collins sucks compared to Boulder doesn't mean you get to bash Boulder. don't be jealous (Ft. Collins isn't bad, but is no Boulder).
Boulder is sick. It is pricey, so get a good job or a trustfund (joke). The town in really nice, close to skiing, hot women, good food, great access to mountain biking, climbing, hiking, etc, and good nightlife (pearl st, the hill) and weather. Plus, just look around. It is probably one of the most beautiful places to live anywhere. I've lived here my whole life and I love it. people who bash it are just jealous. some of the people here are dicks, but that is the case everywhere. If they bother you than your just being a pussy.
Denver is fun. Being 21 would open up a lot of nightlife, but other than that there isn't much holding you back. It has gotten much nice in the past decade as they developed Lodo. For some reason people in CO tend to give it a lot of crap, but I think it a fun place. It a realatively clean, safe, fun city. People complaing about the smog should go to any map metro area - they ALL have smog.
For a combination of Nightlife, living costs, skiing access, and jobs Denver is hard to beat. It may not have as easy access to skiing as SLC, but Denver's nightlife crushes SLC. Job market is still decent too.
Do you know where the job would be? In Denver or one of the surronding cities - ie. littleton, Lakewood, arvada, etc.etc. the cities surronding denver are typical surburban bores. I would probably make a lot of my decision based upon where I ended up.