I live in SEA and travel about 110k/year almost exclusively domestic (except for major vacations). Here is my input on your situation:

AS is by far the biggest carrier in SEA, as it is their HQ/Main Hub. However, as you have noted, AS is very strong on the West Coast but has somewhat limited trans-continental flight options (BOS, ORD, WAS and a few others).

After AS, UA and NW both have many daily departures out of SEA. UA used to have more, then cut back in Chapter 11 in 2002-03, but more recently NW has also filed Ch11 and cut back so I don't know currently which truly has more daily flights. CO, DL and AA all have significantly fewer options out of SEA than UA and NW (measured by both flights and gates) so IMO you probably would be best focusing on those two.

I used to be primary on UA, but really got tired of connection problems in ORD and SFO (for West coast travel).

So for the past 6-7 years, I have focused on NW as primary (with AS secondary). Traveling east you will connect in either MSP or DTW, both of which also provide several daily flights to Schipol (on NW and KLM codeshare) which I saw you listed.

My observations/considerations:
1) I have qualified for Plat each year on NW and depending on the year I have been MVP or MVPG on AS.

2) Between the two, NW requires 75k for top-tier elite, while UA is 100k - for me this made a significant difference.

3) NW provides automatic domestic (48states) upgrades for elite travelers while UA provides E+ and upgrade vouchers depending on your elite tier. Note: SEA is a heavy elite market for NW so unlike other cities you cannot count on an upgrade every flight - this year my upgrade % as Plat out of SEA was probably 75% and many of those were at the gate vs. in advance.

4) NW WorldClubs are reciprocal with AS Board Room, and both are generally much nicer than UA Red Carpet Club.

5) AS will typically provide a status match to MVPG from top (or sometimes middle) tier on another major, NW will typically only match to Gold but not Plat.

What I would probably do in your shoes is to compare my estimated travel pattern and decide if I wanted to focus on Elite status on either AS or NW as primary with the other being secondary. You will get frequent upgrades on both (on their own metal), so you will need to decide which is of greater value to you - I chose to focus on NW since my travel mix is about 60% midwest and east vs. 40% west. I also place a higher value on being upgraded for 5-6hour itineraries than 2-3hour ones. You can also earn on CO and DL for either airline, but AS also partners with AA, NW does not.

Once you've decided you can credit travel on both airlines to your preferred account and once you've earned desired elite tier you can transition to the other. For example, you may find that AS-MVP and NW-Gold is better for you than AS-MVPG and NW-Silver. One thing to keep in mind is that if you are crediting your miles to the program other than that you are flying you will give up upgrade privileges on that segment (there used to be some clever ways around this, but I think the systems have closed all of the loopholes now).

I'm sure there are those who feel that UA is better than NW out of SEA and they may be right depending on their own travel plans, but I will also throw out that my business partner was a top-tier UA flyer for many years and switched over to NW in 2005 and has been very happy with the change.

BTW- there are a LOT more experts available on each of these programs above by going to the appropriate FT board.

Best of luck with your decision and welcome to Seattle!