Stayed at Westin with my wife months back. We were upgraded to a spacious suite but I did not feel furnishings were what I would have expected for a suite (dated). Overall the entire hotel is lovely, staff was helpful, and we received welcome gift wine delivered to our suite the evening of arrival. The breakfast buffet was excellent. There are many nearby restaurants to walk to for dinner. We visited the St. Regis and I felt it more upscale in appearance. If you walk from the train station you walk past the St. Regis on your way to the Westin which with luggage is a considerable bit further. As far as location I don't feel either is much different for walking to the Roman Colosseum and other historic locations. From either I would not walk but take a taxi to the Vatican.