IMHO, the "Ally & Lisa" pic would look good on the front cover, with the heart-shaped collar a nice background image on the back and frontpiece. (But then again, I'm one to talk, having no artistic talent whatsoever... )
The left one! Although the right one is simbolic and stylish, i think it's better to use left cover to intrigue costumers: "Oh, wow, who are these fantastically drawn girls? Is that a comic? Not a book? How it will look like inside? I must buy it!"
OMG I compared this one with the first you made....for God sake, I choose this....God that make all the difference....but leu me ask, could you put the before-after from this image for us?! THAT'S PERFECT!!!!