In my limited experience, some kids test about the same on both the SAT and the ACT while other kids do worse on the SAT. For some reason, the other way around seem less common, although my samples are skewed towards the high end. The ACT is achievement oriented and appears to test basic straightforward knowledge covered in standard high school courses. The SAT is still kind of a hybrid ability/achievement test although the current version isn't accepted as IQ equivalents anymore. The SAT appear to required somewhat more fluid thinking rather than basic knowledge.
This likely doesn't apply anymore but the SAT and the ACT cohort used to be very different. In the 1980's, top schools on either coast did not accept the ACT and many top students viewed the ACT as a basic achievement test.
