I've heard this area called a few different names but I'm going with The Cliffs behind the Art Museum.  Anytime I have a session around the Art Museum area I try to make a point to stop by this spot as well.  Just a little overlook area next to the gazebos that have uninhibited views of Boathouse Row and the Schuylkill River.  Timing out for sunsets give you such a dramatic backdrop that overlooks another iconic spot in Philly.