Just chiming in, but the Westin Copley Place is much better suited for exploring the rest of the city if you're planning on taking the subway. With the Green Line (Copley Station) and Orange Line (Back Bay Station) within a block of the hotel, it's a super convenient property. In contrast, the Westin Waterfront is a a 22-minute walk to the nearest subway station (Broadway on the Red Line).

On another note, my recent stay with the Westin Copley Place was delightful. I was able to check in around Noon on the day of my arrival, and my Charles River view from the 35th floor (free upgrade) was amazing during the day and the night.