Let's inspect their YA covers (first edition, hardcover publication) and see what faces show up.
By Libba Bray:
Published December 9th 2003 by Delacorte Books for Young Readers
Published August 23rd 2005 by Delacorte Books for Young Readers
Published December 26th 2007 by Random House Children's Books
Published September 22nd 2009 by Delacorte Books for Young Readers
Published May 24th 2011 by Scholastic Press
Now Anderson's covers:
Published March 3rd 1997 by Candlewick
Published August 4th 1999 by Candlewick
Published September 23rd 2002 by Candlewick
Published September 12th 2006 by Candlewick Press
Published October 14th 2008 by Candlewick Press
Anderson's book covers feature more faces than Bray's do, though there are plenty of bodies found on Bray's covers. Interestingly, most of Anderson's covers are graphic or illustrated, while Bray's are more photographic. The subjects could have been modeled. (Even the cow looks real.) As with last month's face off, the female author's covers feature mostly female bodies.
Another comparison to make: the representation of history on the historical fiction jackets. Anderson's Octavian Nothing (volumes one and two) covers provide more clues as to the period through the clothes and articles like the mask that Octavian is wearing. Even the muted colors contribute to that feeling. The subjects of Bray's Gemma Doyle series are wearing old-fashioned clothes, but the level of brightness and clarity gives them a more modern feel, in my opinion. These aspects were most likely considered when the publishers were looking at the appeal to prospective readers, but that begs the question of why? Wouldn't young girls be interested in the same story if they saw more cloaks, more artifacts that represented the period of the story on the cover?
In general, Anderson's covers have a lot more details to catch a reader's eye. But, maybe the reasoning is simply related to the audience: girls are usually more avid readers, while boys tend to be more reluctant readers. Publishers don't have to work as hard to entice a girl to open a book. Could the answer be so simple?