I'd have trouble with categorizing Washington. I mean, take King County which shifts from densely urban to suburban to Twin Peaks land as you travel east.
I think Pierce is even harder to characterize. It's got an old >200k urban core, Tacoma, that's both a suburb of Seattle, but also a major regional employment center, tons of suburbs, followed by exurbs, followed by farmland, followed by forest.
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Perhaps a way to help distinguish counties with cores like Rochester and Aurora, IL is to include a non-population determinate, such as employment data. Job density or even total jobs can help differentiate a small urban county with a suburban one.
