In terms of location, I don't think there is a question between the two, especially if the outdoors is a major factor. Boulder is known as one of the best places to live in the US. It's a beautiful place, with easy access to some awesome mountains. The city is funky/cool/artsy; the people are interesting/smart; and it is constantly ranked the happiest place in the US. St. Louis, on the other hand, is thought to be a pretty shitty city (I've never been there so I cannot attest to this.)
And of course, Boulder is near Denver... and Denver is awesome!
In terms of philosophy, though, I'm not sure. That's up to you.