My pick would be someone relatively unknown and about Cavill's age (thought that's secondary). To hell with the established glamour marionettes. They're not hungry anymore. They don't have enough at stake. Even if Affleck would do an ok job as Batman (he won't), he can count on a cushy future. With a newcomer, you can bet he would give his everything to get every detail right.
Get someone who can act, looks like Bruce Wayne, has learned the essentials of Batman's character history and, if possible, is himself a Batman fanatic.