I can only speak to the math side of the ACT. I actually have written practice questions for the ACT for a test prep company near where I live. The ACT is all about knowledge. The questions aren't tricky, but they do involve higher math than the SAT. For example, the ACT covers basic trigonometry, functions, practical geometry but not calculus. I can see younger students who are gifted doing better on the SAT just because they may not have been exposed to some of these other math concepts. The ACT also has a section on science.
