I don't think you can go too wrong. The choice level of the boating here is such that the two crowds basically don't see each other. We already drive past so many great runs to get to another great run. We'll see those guys late in the summer at the border of the three-hour drive time windows, basically at the Ohane and Cispus. Otherwise, there are too many good rivers in a much smaller radius.
For Portland, it is a solid 10-11 month season. We have better proximity to multi-day raft floats. Our traffic sucks, but generally only at the times you would expect it to suck. We can realistically get in an after work lap somewhere if you happen to be in downtown or east.
Seattle has better access to BC, but much worse traffic over a longer portion of the day. You get paid more, but the cost of living is higher. Seattle has a bigger airport with better flights, if that matters to you.
Most of the boating will be to the east, so favor your living to that side.