Alaska Airlines (and Delta) just dropped their prices on non-stop flights to Seattle to compete with a Southwest sale (which ends June 22nd). While Southwest has the advantage of Free Checked / Ski Bag fees, Alaska + Delta have the non-stop flights and availability that runs all the way through Jan / February.
Epic Pass holders can use their pass to ski free at Whistler.
Boston to Seattle runs $375ish during the season — these cheap ski flights save 35% and get you there Non-Stop on Alaska Airlines.
As mentioned above your biggest savings actually come from flying Southwest ($254 RT all-in with bags) — the caveat being that you will take one stop in either direction and need to schedule your trip earlier next season.
That may or may not be worth the $90ish per person bag fee savings depending on your situation.
If you are flying out by yourself I might take the Alaska Airlines non-stop, but if you are going with a crew (or family) flying Southwest should save enough in baggage fees to pay for the condo for an extra day or two.
Early Season / Southwest
As far as early season December conditions? Here's a few more clips from last year for anyone leaning toward the Southwest option.
How to Book
For Alaska Non-Stops (and peak season availability) skip down the page. For Southwest, read on...
Since southwest doesn't allow direct links to specific dates, the best way to go about this is to hit the flexible dates Southwest Low Fare Calendar. From there enter the airport you want to fly from (BOS to SEA) and then select December:
That should land you on the right calendar. You are looking for the flights that are $126 one way on the way out there, and $130 on the way back. Per the above this sale covers dates up to December 19th, so flights leaving / returning after that are WAY MORE expensive (minimum $300 each way).