I think folks generally like the Westin Seattle, with the caveat that it seems a little worn and it is a large convention-style property.
There are definitely more upgrade possibilities at the Westin than the W (which has only a handful of suites). Plus the Westin has water views.
Personally I really enjoyed my stay at the W SEA earlier this year, and was perfectly content with a Kool Korner room.
[This message has been edited by gleff (edited Dec 21, 2003).]