Google Fi is using networks of T-Mobile, Sprint and US Cellular. Of those three US Cellular does not operate in PNW and Sprint's network coverage is pretty bad in general regardless of the location resulting in T-Mobile network being the primary carrier. Google does not have it's own Wi-Fi hotspots either, but the Fi service will automatically connect to any and all open WiFi hotspots. Any phone can connect to those same Wi-Fi hotspots, so it's not really that big of an advantage IMHO
When looking at plan pricing, Fi plans have flexibility but do get expensive quickly especially if you use a decent amount of data. If you don't really use data that much, then it might make sense. E.g. T-Mobile unlimited plans for 4 people are $40 per line vs Fi plans seem to be $20 + $10 for each GB of data used (but max out at $80 per month if you use >6GB, which is not that hard).
The Fi service requires a specific phone making the selection more limited and you have to pay it outright.
So, if you use a lot of data I'd suggest giving T-Mobile a try but if you are ok with the Fi phones and don't really use much data it might be better for you cost wise especially if you need only one line. My guess is service wise you'll end up in T-Mobile's network in Fi most of the time anyhow since you live in WA.
Btw, network coverage (especially T-Mobile) has improved tremendously in previous years but to get full benefit of that you need a pretty recent device. T-Mobile is in a process of deploying new low band coverage layer which will improve coverage even further. So if you are going to buy a new phone, make sure it supports 600MHz...