The ACT has a little more variety than the SAT. The ACT has sections for English, Math, Reading, and Science. The writing section is optional- students who elect not to take it can leave early. If the student does the essay, it is scored from 2-12 and also combined with English in a second score.
The SAT has sections for Math, Reading, and Writing. The essay is not optional, but students can choose to leave it blank and receive a zero. That will affect the writing score though, and bring it down considerably. For many younger students, this is not an issue.
When my son took the SAT, he noticed that everyone began with the essay. After that, some students went on to a math section, while others in the same room began with reading. (He could tell about the math because people pulled out their calculators.)
Some kids seem to perform better on the SAT, while others do better on the ACT.