I prefer the pinball scene in Milwaukee. Seemingly every neighborhood has at least a few relaxed, cozy bars with a decent number of machines in the back room, and there are also a few decent venues that are open to minors.
I don't know Portland as well, but based on what little time I've spent there it seemed like the situation was more similar to what it's like in Milwaukee. I've stumbled across some great old classic machines when I was there.