Regarding the weather, I've lived in both places, San Francisco is much colder in the summer than Seattle. San Francisco, and most of coastal Northern California, barely has seasons.
Though Silicon Valley and the East Bay (Oakland, San Jose, etc) do get warmer, San Francisco, being surrounded by water on 3 sides, just doesn't get hot in the summer beyond a random handful of days. (On the the other hand, it also barely has winter :D)
Bring your jacket(s)!
I lived for seven years in Seattle, and there hasn't been a day since I've moved here that I've missed the weather.
The SF bay gets its summer later than Seattle, but it lasts longer, and the rest of the year is massively more enjoyable. Everyone here complains about the chilly weather in the spring, but a bit of springtime chill is far better than 10 months of gloom.
