The content areas are pretty similar. The consensus when I was in residency was that people who did well on the SAEs (above 90%) tended to do well on the boards, but that the predictive value for people who did poorly on the SAEs was low.
Of the people who did poorly on the SAEs, some took that as a wake-up call, and therefore studied and did well on the boards. Others did poorly on the SAEs, and did poorly on the boards.