There are many similarities between the SAT and ACT: they test the same grammar concepts; they demand many of the same reading comprehension strategies; their math content overlaps significantly; and they are almost identical in total length.
But there is at least one key difference that isn’t discussed enough. On the SAT, two of the four sections are math sections as opposed to on the ACT, on which only one of the four sections is a math section. This difference leads to a straightforward conclusion.
Students who are stronger with math than with English may feel more comfortable with and score higher on the SAT, as 50% of their SAT score will result from their performance on the math sections. On the other hand, students who are stronger in English may prefer and score higher on the ACT.
For the last 20 years, I have told capable English students to go into the ACT with a “chip on their shoulder,” because three-quarters of the ACT comes down to students' English abilities. The ACT English section tests students on the same grammar rules and standard conventions of English every time, making preparation relatively straightforward. Strong English students can learn to score very well on this section. On the ACT Reading section, students have to learn to move very quickly to absorb the passage details and find the answers, but many of these answers are in the passages verbatim. Strong readers, therefore, have an upper hand. Then there is the ACT Science section, which, in many ways, is another reading comprehension section. Students are presented with graphs, charts, tables, and text in the context of experiments or studies; students must learn to efficiently integrate the information. There is very little outside science knowledge that students need to know to answer the questions. Thus, fully 75% of the ACT comes down to a student’s English/reading comprehension ability; this is why I encourage strong English students who have diligently prepared for the test to go into the ACT with an expectation of excellence.
Strong math students, on the other hand, may wish to apply a similar mindset to towards the SAT. As opposed to the ACT, on which Math skills only account for 25% of students' composite scores, a full half of students' composite scores on the SAT is determined by the Math sections. Further, much like the ACT English section, the math sections on the SAT tend to focus on similar topics on nearly every test, meaning that strong math students can often achieve strong scores by mastering a few key concepts.
For many students, the above information can help them choose a test which highlights their strengths and, in turn, downplays their weaknesses. Regardless of students' inclinations, however, the key to success comes down to the same thing: preparation. Learning the key skills, strategies, and concepts can help students refine their abilities, allowing them to put their best foot forward by mastering their the topics they are already strong in while simultaneously improving the areas in which they struggle.