Go with whomever has the best chemistry with Cavill, and whomever best fits the character they wrote. Don't cast the role based on who fits the character relating to the comics, go with who is best for the individual story they want to to tell. That said, it is hard to go wrong with Rachel McAdams, long considered to be the next big actress. Hatheway has stolen some of her thunder, but it still isnt really close. While Kate Winslet seems certain to be the next Merrill Streep, Hatheway will be more Sandra Bullock, while McAdams gets to be Julia Roberts. McAdams possesses rare charisma and versatility.
I would agree-it is going to either Jessica Biel or Rahcel McAdams...Hopefully McAdams, for obvious reasons (Nolan had her read for TDK to), but she has been hesitant to take part in big franchises. Biel really is not that talented of an actress. Everything seems forced, nothing natural (except TCSM, she was excellent in that). Hmmm, IDK. I guess Biel prob. has it in her, but I'd bet McAdams is gar more likely to deliver an excellent performance.
McAdams also seems far more Loisy than anyone else. I.E. I could easily imagine McAdams delivering an Oscar worthy performance with a sound script (like the entire cast of TDK did, for instace)