Not sure how literal to take your question. While commuting (especially only for a few months or even a year or two) isn't as big a deal as some like to pretend it is, I would absolutely avoid a transcon commute if at all possible.

Do you live in SEA? Or just wanting to commute there? If you live there, I'd pick any BES in SEA if offered. Second choice would be anything that has a base there. There really is no "ease of changing later" other than the technically true but radically over-hyped 320-330 transition. That's it, and IMO its not worth a year or two commuting especially transcon if you live in base anyway. If you commute anyway, then SLC or LAX may work for you for a bit.

There is no sense of urgency whatsoever in attempting to force the perfect convergance of cicrumstances. Lots of movement on the way. I'd prioritize desired base over desired equipment.