Overall, 21 percent of commuters who work in Seattle (regardless of where they live) commute by transit, compared to 12 percent in Portland.
Similarly, while 19 percent of Seattle commuters who both live and work in the city commute by transit, just 12 percent of those who live and work in Portland do. (Biking, walking, and telecommuting rates were basically the same in the two cities, and a higher percentage of people carpool in Seattle than Portland, so it isn't that everyone in Portland is just biking instead of taking transit). One theory is that while Portland has invested a lot in transit and bike/ped infrastructure, its job centers are simply too far-flung to support heavy transit riderhip---unlike Seattle, which has a few major, well-defined job centers. Land use, not transit spending, dictates how people choose to get to work.
The numbers get really interesting (and less straightforward) once you dig into the demographic data. In Seattle Sightline found, the typical transit commuter is more likely to be female (25 percent of all city workers, compared to about 18 percent for men); minority (in order, the races most likely to utilize transit in Seattle are: Native American, Asian, black, and white); younger (transit commuting declines steadily through middle age, then flattens out); and/or renters (about 28 percent of Seattle renters commute by transit, compared to about 18 percent of homeowners.) Portland's transit ridership, though lower overall, showed similar trends.
Finally, in both Seattle and Portland, transit ridership declines steadily as income grows, though the impact of income on transit use was more variable in Seattle. Sightline concludes, "looking at the numbers, you might be forgiven for wondering if certain folks who think that we spend too much on transit don’t run in the same sorts of social circles as people who rely on transit to get to work." Another possibility: People might be switching from transit to cars as they gain more income, or vice versa---using their cars less, or getting rid of them altogether, if they start making less.