First of all , as you compare Da Vinci's "Mona Lisa," you might look at technique. Research praises "his treatment of hair, angelic in its fineness, and by the lack of any rigidity of contour" (http://artchive.com/artchive/L/leonardo.html). You might also mention how the use of sfumato is used as a way to show "transitions between tones and shapes" (http://artchive.com/artchive/L/leonardo.html). Thus, it is evident how he blended everything without borders "in the manner of smoke" (http://www.pbs.org/treasuresoftheworld/a_nav/mona_nav/mnav_level_1/3technique_monafrm.html).
I also think the whole facial expression in this picture is both alluring and haunting in tone and mood, making it one of the best admired paintings of all times. What do you think?
You might also comment on his unique use of light in this piece "by the way light falls on curved surfaces. The gauzy veil, Mona Lisa's hair, the luminescence of her skin - all are created with layers of transparent color, each only a few molecules thick, making the lady's face appear to glow, and giving the painting an ethereal, almost magical quality" ...
Da Vinci and Pollack are compared using research.