I am a huge advocate of living in base. If you absolutely have to live in the PNW, then Delta pays the best with the best variety of flying. Will they always have a presence in Seattle? Likely but who knows. United used to be huge up there. Will Alaska? Definitely. If Alaska is no longer in Seattle then Alaska has closed the doors or been bought out.

Every time I fly from SFO-SEA or vice versa I have at least one jump seater. Usually they are very senior. It's a doable commute but you are wasting so many hours of your life. I don't have the commuter mentality. I want to get in my car and drive home, which is why I moved to a base. That being said, your potential earnings commuting for UAL will hands down beat Alaska. I have friends at Alaska and morale is in the toilet there. Most newbies are leaving for the big 3 the second they get the call. I guess my advice would be to suck it up and commute for UAL. Best of luck with your decision!