Although the essay section is optional on both the tests, some colleges still require these scores to consider you for admissions because writing is a skill that you need to hone for your future career even if it is in science or economics. The skills that these essays test are important in college and beyond. Here we will discuss some points that highlight the difference between the two essays.

Time
The SAT essay needs to be written in 50 minutes, whereas the ACT essay needs to be written in 40 minutes. There is a fair amount of reading to be done on both the essays, hence, the additional 10 minutes on the SAT appeal to many students. However, the SAT Essay passage (650 - 800 words) is lengthier than the ACT Essay passage (250 - 300 words) which is followed by three perspectives. So, you need to weigh your comprehension skills and reading speed to decide which of the two tests is more suited to you.
Prompt
The SAT Essay prompt is a lengthy reading passage from an author that presents an argument. You are supposed to explain how the author builds the argument and provide evidence from the text. Your essay should identify, explain, and evaluate the stylistic, rhetorical, and logical elements of the text that contributes to its meaning.
The ACT Essay prompt describes an issue and provides different perspectives on an issue. Your essay should evaluate and analyze the given perspectives, state and develop your own perspective, and explain the relationships between your perspectives and the ones given in the essay prompt.
Analysis
Analysis is one of the most profound differences between the two essays. The SAT essay wants you to analyze a passage that was selected for the quality of its content. Your essay response can be judged accurate or inaccurate based on the analysis you make. Your argument skills play an important role in your scores. The essay must explain how the author - generally a highly accomplished person - accomplishes his or her goals through the use of language and rhetoric skills by picking examples from the passage itself. So, if you can describe why an author is doing certain things to achieve a certain effect, then you are most probably better at doing the SAT Essay.
However, your response to the ACT essay cannot be judged as correct or incorrect. Instead, the essay will be judged on how well you explain your perspective. Your language skills - clarity, complexity, and suitability - of the essay play a more important role than your argument skills. Your perspective must be persuasive and built around a central thesis. So, if you can easily think of the pros and cons of an issue with examples, then you are most probably better off with the ACT Essay.
Outside Knowledge
The SAT Essay wants you to analyse how the author uses language to persuade his or her audience, hence, you do not need to provide outside examples. On the other hand, the ACT Essay wants you to develop your own argument, so you do need to provide outside examples.
Complexity
Both the essays need strong writing skills and, moreover, strong analytical skills. But, does that make the two essays the same? Not at all! The SAT Essay passage is something that needs to be discussed in detail to come up with a well-written response, whereas the ACT provides more freedom to create your own argument.The SAT Essay provides few cues to help you outline your response, whereas the ACT Essay, which is 10 minutes shorter, provides some fixed guidelines, which can create some trouble in framing and completing a response in the allotted time.
So, how do you decide which of the two essays is more suited to you? The simplest method we can suggest is to look within. If you can describe why an author is doing certain things to achieve a certain effect, then you are most probably better at doing the SAT Essay. If you can easily think of the pros and cons of an issue with examples, then you are most probably better off with the ACT Essay.
Also, if you are a student who is well versed with historical and current events, then you are better off with the ACT Essay as it requires you to provide your own examples to explain and substantiate your arguments.
Other titles in this series:
SAT vs ACT Overview
SAT vs ACT Reading
SAT vs ACT Math
SAT Writing vs ACT English