The SAT I is the more commonly accepted test for elite admissions (Ivy League, etc.). Colleges in the Midwestern and Southeastern U.S. tend to prefer the ACT, but overall, the SAT is king.
If you feel that you can do better on the ACT, then go ahead and take it. Colleges make a conversion of the ACT scores to see your SAT equivalency. Why not take both and see on which one you do better, then submit the better score?
Sally Rubenstone knows the competitive and often convoluted college admission process inside out: From the first time the topic of college comes up at the dinner table until the last duffel bag is unloaded on a dorm room floor. She is the co-author of Panicked Parents' Guide to College Admissions; The Transfer Student's Guide to Changing Colleges and The International Student's Guide to Going to College in America. Sally has appeared on NBC's Today program and has been quoted in countless publications, including The New York Times, The Washington Post, USA Weekend, USA Today, U.S. News & World Report, Newsweek, People and Seventeen. Sally has viewed the admissions world from many angles: As a Smith College admission counselor for 15 years, an independent college counselor serving students from a wide range of backgrounds and the author of College Confidential's "Ask the Dean" column. She also taught language arts, social studies, study skills and test preparation in 10 schools, including American international schools in London, Paris, Geneva, Athens and Tel Aviv. As senior advisor to College Confidential since 2002, Sally has helped hundreds of students and parents navigate the college admissions maze. In 2008, she co-founded College Karma, a private college consulting firm, with her College Confidential colleague Dave Berry, and she continues to serve as a College Confidential advisor. Sally and her husband, Chris Petrides, became first-time parents in 1997 at the ripe-old age of 45. So Sally was nearly an official senior citizen when her son Jack began the college selection process, and when she was finally able to practice what she had preached for more than three decades.