That's my choice as well (disclaimer: I live in Seattle). Husky Stadium is really beautiful, in a modern way. The open end of the stadium faces the lake, where the boat owners tailgate. Gorgeous setting.
The tailgating is second-to-none, as you have all of the Microsoft, Amazon and Google folks throwing down serious $$ on their tailgates. One guy has a full bar with stools and 3 HDTVs. Plus you are allowed to return to your tailgate at halftime and re-enter.
As for the game day experience, it's very pleasant. UW fans don't harass opposing fans, even if they're from Oregon. The bench seats aren't cramped because UW actually reduced the number of seats to make things more comfortable. The facilities are very modern with great food and craft brews.
As for the weather in Seattle, it's usually perfect in September: Mild temps and sunshine. The rain doesn't really start until the middle of October usually.