Great posts from Cricket and Slowplay. What they said. Also, congrats on having this difficult decision.
I'll give you my take.
I got out of the navy in early 2009 and waited (flew in the Reserves) for a year and a half waiting for Delta to hire. While I'm from Washington state and it killed me to spend time on the east coast, I never filled out an Alaska application (yes this was stupid but it worked out for me). For the record I'm a huge fan of the people and the culture of Alaska, my sis used to be a flight attendant there. They really do a great job of keeping a positive culture.
Living in base and driving to work is HUGE, don't discount that. And flying the same plane your whole career? Who cares. Going to training sucks. That said, If I was to be stuck in the same plane for a career, I really would not want it to be the 737 (have flown the 88, 757/767, and now the 737 at Delta).
If you had the offer from Alaska, I'd tell you what a fantastic company it is and don't look back. However, since you're in the catbird's seat, I'll admit that were I in your shoes, I'd go to Delta.
Near term, it's looking like you'll get to the west coast in no time with Delta. (Looking at our deliveries coming, I'd bet on you being able to dodge the 88 or 717 as a new hire. Do you have a high last 4?)
Even if Delta closed Seattle, you'll be senior enough soon to hold the right seat of a widebody. Many of these trips are commutable, especially out of Seattle, or start with a deadhead which can be deviated. They also have long trips which are great for commuters. Guys bid the 747 and the 777 in DTW from all over the country. I'd like a 330 guy to chime in here, but don't most of the 330 trips out of SEA leave at like 1 in the afternoon? If so, easy to get to Seattle from Portland for these trips.
OBTW, they keep saying that Seattle is going to see the 350 and more 330 growth there. Yes, the dollar is strong right now but long term, I really see Asia being the source of huge growth going forward. Delta has picked Seattle as their west coast Asia hub. I'm betting on (and do think they will achieve) success in this endeavor.
The surge of retirements coming to Delta is huge. You'll be a Captain soon if you want, or even better an FO on a widebody. A note about this. Judging by guy's bid choices, life is good as an FO on the big metal. I've met guys who were senior enough to hold Captain on every other airplane at Delta save the 747/777, but bid right seat on the 777. I'm on the cusp of being able to commute to a Captain gig... even if they put those EMBs in Seattle, my plan is still to spend a LONG time in the right seat of the 330 in Seattle when the time comes.
Finally, pay. It's not just that you'll make more at Delta. If you're a QOL guy, making more money in fewer days at work (which you will do at Delta) means you'll be able to drop a trip here or there and make the same money you'd make working a few more days at other airlines.
All that said, either airline could go downhill. Prior returns do not guarantee future results, etc. Congrats on doing so well in your career that you've got offers from two great places to work. Best wishes and feel free to PM for more info.