Personally, I'd take the Westin. I prefer the view; I like the canal instead of the lagoon. And at the Danieli, the view includes the extremely busy pavement in front of the hotel. Since you're willing to pay for the level room you want, and since the canal view balcony is guaranteed, you should be assured of a great view at the Westin. It really is magical, sitting in your room (on your balcony) looking at the canal traffic.