First of all, since DL has cutback so much service out of PDX, all airlines are about equal at this point. Next, if you fly to DCA, you will incorporate one more stop enroute. At least with IAD, you can fly nonstop with several carriers- UA/AS from SEA.

I would be partial to NW, good connections and partnership with AS so more choices connecting PDX-SEA. You could even use CO and DL for variety of connections through NY as well. And as Platinum, you get 125% bonus miles so if you fly that often, it would be worth it. As NW Gold, I always got upgraded on NW flights which has good food and service on 757 SEA-DTW. CO however is another story but have good success rate in off peak travel, ie. on a Tuesday midday or early Saturday morning.

If you choose UA or AA, you limit yourself greatly to a handful of flights and basically no redeyes out of PDX, except for terrible connections at 5am through ORD. Plus you will not see the front of the cabin as much as you would on NW. Any other airline would be a waste of time, ie. Air Tran, Southwest or America West.

Lastly, you don't say where you would be "living" when you are here. I would pick IAD and take bus to Rosslyn. All major hotel chains are represented in Virginia suburbs and provide better value than DC hotels, in terms of cost, size and public transportation location. You can walk across bridge to Georgetown for dining and nightlife too.