I would pick the Sheraton mainly due to its location specially if a leisure trip. While the Westin is really nice, it is a little far from restaurants and such, but not terribly far - less than a 10 zloty ride from old town and around 20 to the Sheraton neighbourhood. I tried restaurants at both places and the one at the Westin was a little better in my opinion. The Sheraton is an older, more established hotel.