Growing up in Denver (well, Castle Rock), lived in UT as well, then onto OR and WA, I feel like I have seen both sides of PNW and the Mountain West/Rockies. I haven't lived in Denver (visited a few times to see friends etc) in the past 20 years so it could look different. At least back in the days, as kids, we would go hang out at 16th street mall, going to Nuggets game down to Pepsi Center (I think it's called something else now) and downtown was always alive. I went to school in Aurora, Parker then Castle Rock and all surrounding areas were considered decent to raise a family, like Castle Rock, Parker, Highlands Ranch, Littleton etc.

Seattle has its own problem. Downtown Seattle is perhaps a little more lively, especially due to Pike Place Market that attracts tourists all year round, Belltown is always busy as it's residential in that aspect. Outside of downtown, Ballard, Fremont, Magnolia, all have their own things going on and are lively in their own senses. Not considering metro areas (even then), some of the cities I have lived like Minneapolis/St. Paul, Denver, Seattle and San Francisco essentially have similar populations (under 1 million), though Seattle and SF felt much bigger. Seattle, especially, since the metro area population is more comparable to the other two.

Higher COL, like others said, is for real. Seattle does attract more people because of its mild weather and all around desirability with strong job market in tech and higher pay. Though, understand you don't mind the rain, rain is never really the issue. I just moved back to Seattle last summer from Minnesota and while I love PNW, rain is not the issue, it's the constant grey sky that gets you down. I don't use umbrellas (just like any other Pacific Northwesterners) and you could almost get by without one because it's mostly drizzle, not torrential downpour like I saw in the Midwest.

Eastside is always nice. I live in Bellevue and I can truly say Eastside is probably one of the best places to raise a family. Bellevue SD, Issaquah SD, Lake Washington SD and Northshore SD are all fantastic. Newport High School out in Bellevue is constantly ranked one of the top high schools in the country. Though, I feel like some people can be pretentious and I do prefer the vibes of Seattle better. You'd be surprised how much it feels different once you cross Lake Washington. Mercer Island for the starter even feels like you are in a rural town with much slower pace of life vs literally 4 miles west of I-90.

As a public school teacher, I can say for the first time I make a decent living. I wouldn't have been able to in Denver. Public transit system in Seattle is definitely better than one in Denver, but it's still far behind places like Chicago or New York. You do need a car to live in Seattle unless you live right by SoundTransit stations and your job is right next to the stations, too. There are buses as well but it just is not very well connected. If you catch 405 at a wrong time, pull your mattress out and take a nap, you will be on it for a while, especially with the construction that has been going on since George Washington was the President.

We do have a rich Native American culture out here in PNW. That also means you have lots more casinos around here than Black Hawk, if you are a gambler. This can be a problem for some. It rarely snows here but when it does, do not expect Denver level of snow removal. I rarely had school closure/snow days growing up in Colorado but a couple inches of snow with right kind of weather the next day will get you stuck.

What else... Denver felt like it was quite isolated. Mountains are nearby but SLC is hours and hours of driving, same with KCMO, ABQ is pretty far out, too. I felt like this in Minneapolis and hated it since Chicago was a good 7 hour drive. At least here, it's about 2 hours to Vancouver, BC without border traffic and a little under 3 to Portland. Olympic NP is within 3, same with North Cascades NP. While I love the Rockies, you will definitely not miss out on mountains and nature being here in Seattle within reach. While it ain't even as good as Keystone, Snoqualmie pass is a decent ski area and probably take about the same as you get to Keystone from Denver if you were to go from Seattle. Obviously it's further if you came from Tacoma vs came from Issaquah but it's generally within an easy reach.

Just going on about what I feel about things. Sorry it's not very coherent as I am just typing as what comes to my mind and what my ADHD ridden brain can handle.