So, I live in Portland, a few thoughts... Portland is definitely cheaper by degrees and there are a few factors that play into that. Living in a decent area of Portland is much cheaper than living in a decent area of Seattle, and there really are hardly any bad areas of the city of Portland itself, except for the most ghetto depths of NoPo. Portland is a much more compact "European" city than Seattle. You don't really need a car and it's easy to walk from place to place, a real city of neighborhoods. Seattle is much more spread out and one has to deal with traffic, and I personally feel is not all that attractive of a city on a whole. But it's bigger, and all that entails.
As for taxes, if that's important, Oregon has higher income and property taxes, but no sales tax. I don't think it would make too much of a difference if you were a full time student, and is nothing compared to, say, California. If you are of a libertarian bent and are angered by the Guv'ment taking your hard earned dollar then you can live in Vancouver, Washington and have the best of both worlds, but if you are of a libertarian bent then you probably shouldn't go to either of these schools. And if you want to live in Vancouver, Washington you should probably get a crack habit too.
If you guys have questions about Portland I'd be happy to answer questions about the City of Roses or anyplace in the northwest. I spend a lot of time in Seattle, Eugene, etc.