Well, I'm very familiar with both, but without knowing a lot more about you, I can't really advise you.
Seattle has become more and more expensive and is really attracting people with significant education and skills who can afford the place. I have relatives there who just retired and who sold their small house for an enormous amount of money -- about 10 times what they paid for it in the 80s.
Denver's not cheap, but real estate prices and rents are more reasonable. When my father-in-law who was living in the Denver area (Aurora) sold his house (to move to Seattle and join the relatives I mentioned), he got far less than the relatives just got for theirs, even though the house was bigger.
In addition, the climates are different: Seattle's is cool, moist and somewhat gray much of the time, It rains a lot. Denver's is hot, sunny, and usually dry, though it can get hit with sudden snowstorms even early in the fall and late in the spring.
You don't say what your interests are or what field you work in, so I can't say much more.