Most universities and colleges in the United States accept scores from either the SAT or ACT. Before you choose which test to write, check which test the schools you're applying to prefer. If they only accept SAT scores, for example, that will make your decision a lot easier!
No matter how much you want to avoid adding one more decision to the whole process, being able to choose between the ACT and SAT can give you a distinct advantage. The two tests are significantly different from one another. They test different skills, which means that depending on your strengths and weaknesses, you may score higher on one test than you will on the other.
Here are a few of the differences between the tests:
The best way to choose between the two tests is to do a bit of research. Take a practice test for both the ACT and the SAT, keep track of which types of questions you found easy to answer and which ones you struggled with, and compare your final scores. The key to a high score is playing to your strengths. If your vocabulary isn't great but you are an expert problem-solver, go with the ACT. If you'd rather play with words and avoid math, you're probably better off with the SAT.
One final thing to remember: standardized tests are not intelligence tests. They are used as a common measurement to compare students from all over the country, all of whom have had a slightly different education. Since no test can accurately take into account all the differences in test-takers' personal circumstances, the results are often skewed in favor of one group over another.
As stressful as they are, try to keep the tests in perspective. While a low score might mean you don't get into the Ivy League school you were hoping for, it is not an indication of lower-than-average intelligence. Test scores are only one aspect of many admissions criteria used by colleges, so don't give up your dreams too soon!