I'd hate to say I'm promoting UA because of obvious reasons, and especially in a NW forum, but given that your main base is WAS and you do fly to Europe and Asia aside from WAS-PDX, it seems that UA is a better bet for you.

Reasons:
1) WAS-Asia are one-stop on UA to PVG/PEK/HKG vs. two-stops on NW.

2) UA has a big hub at IAD which can get you to many destinations domestically non-stop. Seems like NW would require transit in MEM/DTW/MSP - especially to the west coast where UA has IAD-LAX/OAK/SAN/LAS/SAN, etc.

3) As for Europe (if UA flies there), UA has nonstop service from IAD. If UA can't get you there, a one-stop connecting on LH via MUC or FRA will get you to all of Europe. I am not sure how NW would work to Europe from the east coast without flying on KLM only.

In any case, it all depends on your needs - note I haven't mentioned a thing about service, which to many people are secondary to flight schedules and network from their home base.