Assuming that this is business travel you're talking about, have you checked with your company's travel office to see if you have a preferred carrier that might grant a discount or complimentary elite status?

I'm also out of SEA and have chosen UA for a few reasons:

1. Company preferred carrier with a big discount.
2. Most nonstops to the destinations I travel to most often (SFO, IAD).
3. Best worldwide route network of any US airline with easy and frequent connections to pretty much anywhere I'd like to go. Star Alliance covers everywhere else.

As kempis pointed out, the SK nonstop to CPH is nice because it provides just about the easiest and fastest connections to most anywhere in Europe from Seattle.