The best way to #prep for either tests is to take a practice test and score it and analyze your results. The tests cover the same subjects but the layout and questions are significantly different.
Similarities: Both tests are 3 hours in length. Both have a Reading section and Math sections.
The #SAT calls the grammar section Writing and the ACT calls it English.
The SAT has a No Calculator section and a Calculator section. The ACT Math section is all calculator.
The #ACT has a separate science section but the SAT has a science component within the reading section.
While both tests are the same length in total, the ACT is more time sensitive. For example, the Math section is 60 questions in 60 minutes which is extremely tight. Therefore, there are strategies to make the best of a score by knowing which questions to complete and which questions to answer but not spend time on.
Do colleges prefer one test over the other? No
If a college or university is test optional, should I take the tests? It depends.
For smaller schools that look at the applications holistically, tests may not be critical.
For larger schools that are now going test optional, test scores are a way to separate students and make automatic decisions for the higher achievers. For example, if a university receives 60,000 applications, they do have a system for categorizing students. Tests even for test optional schools are still valuable.