I went to graduate school in Seattle and my wife's family is from San Francisco. Comparing the two areas:

Culturally, SF is tops. It is a world-class city, with great drama, music, museums, libraries. It's been said that, of US cities there are San Francisco, New Orleans, and New York -- the rest are all Newark. The climate is Mediterranean and mild -- it is a great place to be outside, year-round. A 3-4 hour drive gets you up to Tahoe. Driving can be a hassle in SF, due to limited parking, so a bike is often.

Seattle is an up-and-coming city, but not quite San Francisco yet. Seattle Opera is pretty good. It does rain a lot of the time from September to March, although it is usually such a fine mist that rain gear is not required. If you want to dive, ski (XC and downhill), hike within a short distance from the city, Seattle wins. San Francisco is good, but Seattle is better in this respect. A two hour drive gets you into the Cascades. Bike commuting has been well established in Seattle for at least 30 years.

Seattle has an upscale crunchy-grainola feel to it, where SF is more sophisticated and bohemian.

Paul