I love both of these characters, but far too many creators seem unable to tell the difference between the two, which has led to wildly inconsistent characterization whenever they appear.
Ms. Tessmacher first appeared in Donner's Superman as a fun mix of the traditional gangster's moll and wise-cracking ball buster whose ever-shifting morality becomes a major plot point in the 3rd act.
Mercy Graves first debuted in the 90s Superman cartoon and was a more straight-laced Bond henchwoman to the Blofed-esque Luthor. She had no moral quams about her duties and was ruthlessly loyal to her employer.
However, ever since Smallville's Ms. Tess Mercer threw both into a blender with Lena Luthor, these two characters seem to have become interchangeable, depending upon who is writing the story.
Given the current vagueness of DC continuity, how would you like their respective characters to go in whatever comes next?