Between the Westin and Sheraton in Downtown Seattle, I prefer the Sheraton without a doubt. The Westin is old and in dire need of a remodel. Since you will be in Seattle in Sept, the Sheraton is opening their new addition (400+ rooms) next month (June), so the issue of older rooms is no longer an issue. Unfortunately the parking fees at either properties are quite expensive ($25-$35). No experience with the W in Seattle. I am sure if you did a search in this forum, you may find more information.
Have a good trip.