Suggestion: if you know what destinations you will be going to and how frequently you will go to each one and what kind of schedules you will be going to them on, do some flight shopping to get an idea of which airlines have the best service from SEA for those trips in terms of schedule convenience and such factors.

Then plug the trips into the frequent flyer programs you are considering, to see which one will give the best status and awards that you consider to be desirable.

I like AS' program, but from an award, not status, standpoint. Lots of partners to collect mileage on (AA, CO, DL, NW), and lower mileage threshholds for award flights on AS planes than other carriers (20,000 vs. 25,000 for a US domestic economy ticket, for example). Plus award flights are easy to find. But you'll have to ask others about AS' status usefulness.