The ACT has been outpacing the SAT in the U.S. in recent years.
Check this graph we put together:
However, this past month, Michigan announced they are replacing the ACT in their schools as a High School requirement.
Well, at least for 3 years, Michigan will now persuade their students they need to be more proficient at the SAT. Interestingly enough, the NEW SAT is to change in March of 2016.