Gimme a defensive HC every single time.
Your coordinators create your game plan, your HC instills attitude. Defensive minded HCs create a more cohesive, aggressive attitude throughout their entire team.
It seems to suggest I would be pro-Cowher but ultimately I went Billick. I know his background is as an OC, but he did a great job with his defenses in Baltimore and really showed that he knew how to operate on both sides of the ball.
Ultimately I wouldn't want any of them, but if forced to pick between those 3, gimme Billick.
If you can't draw a crowd, draw dicks on the wall.