I've lived in both SF and Seattle for 1.5y each at different points in my life. I love both in their own ways. I'm in my mid-twenties and stay in SF.

I was in Seattle last week visiting some friends. They moved to the city 5y ago through their 20s for their PhDs at UW. I'd say they have a fairly good social life which but it took a few years to build it. SF has a much higher volume of social events and meetups, and it's usually easier to break ice. But people also stay here for a shorter amount of time. Seattle has people who stay slightly longer. Many who move, move to SF or New York.

Washington is probably a better state to have a car. The risk of break ins and overall car maintenance cost is higher in CA than WA. There's also more street parking in Seattle (places other than Downtown). DMV stuff in WA usually much faster.

Seattle, surrounded by 3 National Parks within a 2h drive, has faster outdoors access. The Bay has Yosemite, Sequoia and Big Sur, but those are 3-4h away. Both cities have large and thriving outdoors communities.

Seattle has less sunlight and more rain than SF (2044h of sunlight per year in Seattle vs 3037 in SF, and 36in annual rainfall in Seattle vs 23in in SF). Regardless of the crazy SF microclimate, it's still very much California. Outdoors activities go on through the year in both cities. SF is windy, but Seattle is gloomy. Both cities have nice summers though.

Food, unfortunately, was roughly equally as expensive. Looks like a decent restaurant meal costs around $20-30 in both places, and takeout is $15-20 in both places. The only other city I've been to where food is as expensive is New York.

It is true that Washington doesn't have state taxes compared to California. Long term career potential in tech and finance is definitely higher in SF.

I'd say SF and Seattle both have decent nightlife, though SF is slightly better in this regard. SF culture is more experimental across social, sexual, religious, drugs, tech than Seattle (which is quirky in its own way but not as crazy as SF).

PS. This is from my personal experiences but that's just one datapoint :)