Personally, I prefer the Westin. I stayed at both earlier this year. The interior there has been refurbished, and the views are really nice, especially if you get a high floor room. Sheraton looks a bit tired and more old school (surprising, since Westin is way older, and looks older on the outside). Location-wise, both are on the fringe of the city center, Sheraton is a bit more central as it's closer to the train and bus stations, but it really doesn't make that much of a difference.