No need to prep for both. Virtually every college accepts both ACT and SAT equally and use the concordance tables to compare the score of one to the other. So all things being equal, it is to your advantage to choose the test that gives you your best advantage.
The ACT has a science section. Even though the ACT science section does not really require previous knowledge of science, it does test your ability to quickly but accurately interpret data. In other words, the science section Both tests come with optional essays. The ACT essay asks you to come up with your own argument and support it – the New SAT essay asks you to evaluate an argument that someone else has already written for you. Neither is easier or harder – it’s just an issue of personal preference
The use of a calculator is limited in the New SAT. The ACT lets you use a calculator on all its math problems, and all the answers are multiple choice. The New SAT has a “with calculator” section and some answers are fill-in. The “without calculator” problems do not require any difficult arithmetic, so it’s not that much of an issue. And just as before, if you are a strong math student, you will find the more in-depth math portion of the ACT will work to your benefit.
The ACT is fast. It averages to about a problem a minute. This is great if you like to make your choice and move on. However, if you tend to mull over the questions and reach into your higher order thinking each time, you may run out of time on the ACT and might do better on the SAT. Prep for the ACT is skewed more heavily toward time management than prep for the SAT. The pace of the ACT vs. the SAT is probably its biggest difference.