What makes the two tests more similar, other than the number of sections on both the tests, is the similarity between the SAT Writing and Language section and the ACT English section. The two sections have more similarities than differences. When the SAT changed the test structure in 2016, the SAT Writing and Language section underwent significant changes. The Collegeboard made it very similar to the English section on the ACT. The current SAT Writing and Language section uses the same passage-based format as the ACT English section. However, despite the heavy similarities between the two sections, there are some noticeable differences too.

Number Of Passages
The SAT Writing and Language section requires you to comprehend four passages, whereas the ACT English section requires you to assess five passages. More passages means more information to sift through. Hence, the ACT is more suited to students who can read faster and more efficiently, whereas the SAT is more suited to students who prefer to read slowly but more analytically.
Number Of Questions
One of the most apparent differences between the two tests is the number of questions on the two tests. The SAT Writing section contains 44 questions, whereas the ACT English section contains approximately double the number of questions i.e 75 questions. The more the number of questions, the greater the chances of making an error. However, keep in mind that the marking scale is different too.
Time Per Question
SAT Writing and Language has 44 questions to be answered in 35 minutes and ACT English has 75 questions to be answered in 45 minutes. Hence, you have 48 seconds per question on the SAT Writing and Language and 36 seconds per question on ACT English. Less time per question can equate to more errors. However, you should not take this to mean that SAT Writing and Language is easier! Some of the ACT questions are more straightforward than their SAT counterparts. If you have strong English language skills and do not overthink, the ACT English section is most probably the better test for you. But, if you like to think in more detail about each question, then the SAT Writing and Language secion is a better fit for you.
Focus Of The Test
The SAT Writing and Language section is slightly more inclined towards questions that deal with writing style and passage structure, whereas the ACT English is more inclined towards questions that deal with grammar and idiomatic language. The ACT English also has the Main Idea question or a question that involves the complete passage, which rarely appears on the SAT Writing and Language section. An example of such a question is "Suppose the writer were to write an essay about how a single discovery in science can have far-reaching consequences in terms of technology. Would this essay fulfill the writer's goal?" However, the Main Idea question is asked on the reading section of both the tests.
Charts, Graphs And Tables
The SAT Writing and Language section has passages that accompany charts, graphs, and/or tables, which are absent on the ACT English section. These graphics do include numbers, but you are not required to do any calculations - calculations are restricted to the math section. Questions related to these tables or graphs would need you to find some patterns or trends to reach a general conclusion.
Idioms And Vocabulary
Compared to the SAT Writing and Language section, the ACT English section is focused more on vocabulary and idioms. However, that’s not a point to worry as the vocabulary on the ACT tests basic knowledge about word replacement and understanding of how words and phrases that sound alike can change the meaning of a sentence.
Clearly, these two very identical sections on the SAT and the ACT differ from each other in many ways. Hence, it is important for you to decide which is a better fit for you. You should take into consideration the points above, your comfort with graphics, your reading habits, and the pace at which you work to identify the test that is made for you. The SAT vs ACT test from Collegify will help you get started. The next step should be a full-length diagnostic. Here is the link to the SAT vs ACT test from Collegify: https://collegify.com/sat-vs-act
Other titles in this series:
SAT vs ACT Overview
SAT vs ACT Reading
SAT vs ACT Math
SAT vs ACT Essay