OMG, no contest: PacNW, hands down. (Especially Washington from Snoq Pass, north.) Thus, Portland. Great city.
Don't get me wrong, Colorado is great, but for backpacking Colorado's mountains wouldn't even make my top 5 western states. For me, Washington would be #1, Mont / Wyoming battling it out for #2/#3, California #4, Idaho #5, with Colorado maybe edging out Utah for sixth.
While I don't care for Oregon's mountains nearly as much as Washington's, (Wallowas don't count, I think of them as Idaho, since travel-wise they're part of the Boise / Spokane scene), at least in Portland you have decent access within a day's drive to the 'real' Cascades (Wash), plus the redwoods, amazing coasts, Trinity Alps / Marble Mountains, not to mention the gorge, and Eagle Cap. Oh, and the fantastic Olympic peninsula is not far away.
I lived in Denver, and yes I like it there (I almost moved back there last year). I lived 2 decades in Utah too. I know this will incur the wrath of Coloradans, but most Colorado mountains consistently disappoint me compared to North Cascades, Northern Rockies (Wy, Mt, Id), or Sierras. Too rounded, too barren, not enough verticality or granitic jaggedness. (Yes, I know the Needles in the San Juans are that, but they're the exception for Colorado, and you're already driving 5-7 hours to get there from Denver. In the same driving timeframe from Portland / Seattle, you have 20x as much of that available to you.)
And the winter weather in Portland is way better than the front range. I'd move from Denver to Portland in a heartbeat without ever looking back.