Directness of QuestionsThe SAT formats questions in a manner that can often result in writers having to re-read questions extensively in order to be 100% sure what they're being asked. This can be time consuming for some, which isn't a good thing considering the SAT is timed. ACT questions tend to be far more direct and after a first read students can quickly move on to answering.
Timing & Nature of QuestionsOne direct difference between the two test formats is that the ACT is 2 hours and 55 minutes in its base format, with the SAT lasting 3 hours and 45 minutes. There is an optional 30 minute writing test that can be added on to the ACT, and while it's optional many institutions do require this writing section.
Aside from the optional writing test the ACT is also completely multiple choice, whereas the SAT incorporates far more written components.
Structure of ContentThe ACT breaks its four main content areas (English, Math, Reading and Science) up and you address all of the questions related to one area together. The SAT, on the other hand, is broken up into 10 sections that cover the main content areas (Critical Reading, Math, Writing) and the sections are intermixed. So an SAT taker may find themselves doing math, writing, math, critical reading, math, etc. and jumping around a fair bit. For students that struggle when moving back and forth between the main concepts may prefer to take the ACT.
Topic DifferencesThe ACT includes a science section (which the SAT does not) and also tests subjects in the math section that are not covered in the SAT. The SAT, alternatively, has a far more extensive focus on vocabulary, though this is offset by the ACT including grammar. This may sound like the ACT is harder, but many disagree as the straight-forward nature of the questions make addressing the additional content far easier.
Scoring and EvaluationThe SAT provides scores for math and verbal each on a scale of 200-800, with the average of all test takers being a score of 500. These two scores and their combined total are what schools will evaluate students on.
The ACT provides 12 separate scores, the most important of which is the composite score that rates students on a scale of 1-36 based on their performance, with half of all test takers falling in a range of 17-23. This approach works better for students that greatly excel in some areas but are weaker in others, as they can still achieve a high composite score and that is what schools focus on.