ACT includes a science section which I believe helps to more thoroughly test high school students. While the SAT is a little more wordy and can be harder to understand. In the SAT each subject broken into smaller sections that aren't consecutively taken. The ACT is definitely the better choice for testing high school students and preparing them for college.
The ACT is much easier to understand and interpret. With its four subjects split into four sections it is much easier and less confusing for high school students to understand the ACT.
Also the ACT is the test