I switched from aa to Delta about a year before leaving PHX for SEA and I've been satisfied with the carrier. Granted, for me it was the bottom tier comparison, but Delta even having to connect to hubs was a better experience from Phoenix, and now I'm at a hub which is cool.
Do you travel internationally very much? Alaska has some good partners, and especially if you fly business class the earnings and redemption can be great.
Anyway, the issue for you is that PAE to SEA can be a major hassle of a commute, but if you choose Delta, grab an Amex Platinum card for Skyclub (and Centurion) lounge access.