I've lived and worked in both.
Seattle has terrible traffic. No, seriously, TERRIBLE. Mass transit is a joke and the city is built on a pretty darn steep (and narrow) hill, so biking is... a lot of work. Housing is outrageously expensive if you want to be close in to the city center - and even a few miles out can add 30+ minutes to a commute. The Sounder Train runs through random suburbs (instead of along the main I-5 freeway) and is on actual train tracks, so it is a) slow and b) only runs a few times a day. It also closes a few times a winter due to mudslides on the tracks. The light-rail is very underdeveloped (unless you want to go from the airport to downtown). No state income tax, which is great. Sales tax is pretty high (over 9%) but if you don't buy stuff, you don't pay it. :) Seattle has a reputation as a "cold shoulder" kinda place - people are more standoffish, or so some folks say.
Portland has a well-developed mass transit system (MAX) that funnels into the downtown core, so your options for (less) expensive housing and a simple commute are larger. More developed bike infrastructure, too. Fewer jobs and there is state income tax. No sales tax, though. Portland has a different vibe than Seattle. More environmentally friendly, accepting of different folks, and a bit more friendly. You'll find more jobs in cool fields, like renewable resources and small interesting companies. The downtown is cleaner and more walkable, designed a bit more pedestrian-friendly. Graduates are moving to Portland in droves, which means there is more competition for entry-level professional jobs and cheap housing.
Portland also gets more daylight during the winter. Not by much, but if you're coming from a non-grey climate, you may find it really hard to handle the endless days of over-cast and the short days in the winter. A lot of people can't handle several months straight of thick clouds and intermittent drizzling rain.