Re: Storm VS Mystique
Storm will definitely be 2nd female lead behind Mystique. Lawrence is the most in-demand actress around at the moment thanks to The Hunger Games and her Oscar win.
As for overall, I'd argue Halle is more marketable than James McAvoy, Patrick Stewart and Peter Dinklage. I wouldn't have said this a month ago, but The Call just crossed $50m in the US and its success has given Halle some momentum. In the right film Halle can carry a movie with her name alone.
If I was part of Fox's marketing department and had to put 4 characters on a poster I'd be Wolverine, Mystique, Fassbender's Magneto, and Storm. Beast and McKellen's Magneto could easily fit alongside them.