I have lived and worked in both Seattle and SV, am currently a founder living in Seattle, and am at this moment writing this from Coupa Cafe in Palo Alto. The business culture in the valley is more aggressive and the community knowledge about the informal processes at successful startups is stronger. In Seattle the community is more insular and there's a noticeable "us vs SV" complex. Having said that Seattle is a good place to build a company, just know that as a founder you are likely to spend a bunch of time in the Bay area.