I honestly cannot think of a single thing that Seattle has that Spokane does not unless you think larger class sizes is a perk. I mean, sometimes having a bigger class can have unique perks like more interest groups or more opportunities to make friends, but that isn't something that you can't do at another site. If an interest group doesn't exist, then just make it yourself.
I have friends at the Spokane site that have done research there, shadowed advanced specialties and matched into those competitive specialties (the most notable is one that I specifically know is attempting to match into neurosurgery and he has done everything at the Spokane site, including his research and first two years of medical school and many of his third year rotations). You would not believe the number of people that say they want to be at Seattle because they want to do a competitive specialty and think that being at another site will hinder their application or that they need to make good impressions in their first year of medical school. This is not true one bit but people still insist that it is.
I am a second year now and have gotten to see my WWAMI classmates come to Seattle and there is not a single difference in our education, and people from all sites have done research and our medical school CVs reflect the effort that each person has put into it.