Art Directors v. Senior Graphic Designers

Some time ago a friend asked me what I thought the difference between an Art Director and a Senior Graphic Designer was. Initially I had no response, but then after a thinking it over I began thinking of people who I know that have each of the titles. Comparing and contrasting between the individuals. This is a list of the few things I conjured up.

A.D.s:
are conceptual thinkers
ignore type somewhat
will photoshop the hell out of a photo

S.G.D.s:
are organized
can tell you from across the room when 2 letters set in 10pt needs kerning
will make type the imagery driving the idea

This list is short and has many gray areas. Anyone else have any ideas on any differences?