more SEA PROs: 1) No state income tax 2) Very Green/Environmentally Friendly (until Seattle gets overcrowded) 3) Perfect Summers (beats anywhere in the country)
more SEA CONs: 1) It does not RAIN a lot. It drizzles a lot, but recent studies in the last decade have shown that Seattle's average rainfall in the given year is ranks like 46th outta 50 US states. So if you don't mind daily drizzle, then ya fine. 2) SEATAC airport is not a major hub, so expect inconvenience and pricier tickets if flying to most major cities around the continental US.
Although you can make arguments on the pro's/con's of any given city. Seattle makes more sense in that its lower overheard for your startup. When you hit it mega-big, then you can live in both areas and fly daily for lunch in SF and dinner in SEA.