Should I take the SAT or the ACT?

I am a Freshman that is applying for a summer university program for High School students (Duke's TIP program). With my application, I have to send in test scores from either ACT, SAT, PSAT, or PLAN. I haven't taken any of these tests before. I have only taken the EXPLORE test (I got a 22/25 composite) so I don't really know how to choose between either ACT or SAT. Considering I am only a Freshman and I am on block schedule (4 cles a semester), which test would best suit me? I have completed Algebra 1 and Physical Science. Taking World Geo and English right now. Are they both multiple choice? My strengths are math and writing. Please help. Thanks.