Neither test is harder — but one might be right for you.
My Advice
If you're undecided, just take a practice test of both. You can take one on your own, timing yourself, and see your results. No need to be in a proctored environment. If you do better on one than the other, take that one. Otherwise, go with whatever one you like better.
What makes the two tests different
Both tests are accepted by virtually every U.S. college, and colleges have no strong preference between them. Objectively, they’re equally difficult, but you might prefer one over the other.
The SAT tests far more vocabulary than the ACT does.
As of the March 2024 exam, the SAT has questions that specifically test vocabulary. Reading and Writing is tested over two modules; both consist of vocabulary, logical reasoning, and Standards of English questions in that order. However, the SAT now does not test multiple questions over long passages; each question is confined to a single short text prompt. The ACT, in contrast, does not explicitly test vocabulary but does have multiple questions each on four long reading passages. Having a strong vocabulary obviously will help students on the Reading and English portions of both exams. However, if your child has a very weak vocabulary, the ACT will pose far less of a challenge than the SAT. On the other hand, if your student is a bit of a slower reader, the SAT format and more time per question might be a better option.
The ACT has slightly more complex math, covering a much broader array of topics.
Both exams require strong knowledge of arithmetic, algebra, and geometry. The ACT requires more than that though, covering topics such as trigonometry, matrices, imaginary numbers, 3D spatial reasoning, logarithms, quadratic equations, and more. However, the ACT math questions are definitely more straightforward while the SAT questions are far more "tricky." Thus, I don't believe that either section is more difficult than the other.
On the other hand, Math is only one-third of your overall ACT score, while it’s an entire half of your SAT score.
If you don’t like reading long passages, that’s one factor in favor of the SAT.
The SAT has done away with long reading passages. Now, instead, its reading comprehension is tested mainly by questions over short paragraphs and questions over a list of bullet points. Short-paragraph questions ask you to find the main point, a logical conclusion, what might strengthen or weaken an argument, the function of a phrase, or to analyze or use data in a graph or table. Bullet-point questions (synthesizing information) ask you to use the points and present some specific information therein. The ACT, on the other hand, has an entire third of the test ask about four long passages, with questions about the general themes of each passage and specific information within it.