SV's concentration of investors and startups is unparalleled. There's strong evidence to support this beyond qualitative assertions anybody may make.
But I believe that SV/SF is hitting against constraints. Of course history shows us during gold rushes, there's plenty of people willing to suffer poor conditions to pursue the perception of easy riches.
Seattle has a big limitation for startups because of the lack of ready angel investors. Any startup beginning in Seattle and unable to close a notable angel investor have a hard time raising an angel round. Though SV investors and VCs are showing a willingness to support early stage Seattle startups. A16z has made investments in Offerup and Distelli.
There is a lot of high quality engineering talent available in Seattle. Though many tend towards conservative choices for careers. Recruiting engineers into a startup requires some work. Though once a team's built, there's not the same velocity on job turnover that's typical of SF/SV.
Also for recent grads, Seattle offers a great lifestyle that shares a lot of attributes with SF and Portland. Housing is of high quality and available. Thanks to Modernist Cuisine by Nathan Myhrvold, many rising culinary stars are visiting Seattle in a form of a pilgrimage.
There's a lot of promise in Seattle. Many in the community are dedicated to growing the startup ecosystem. New Tech is a good meetup with great organizers in Red Russak and Brett Greene. I have nurtured my Python user group PuPPy for a year now. We're approaching 1000 members and hold monthly meetings that draw well in excess of 100 attendees at every event.
It's a personal choice. Like choosing a school, I'd recommend a visit. But I'd recommend making several trips over a period of time before making a commitment to either destination.