Traditionally, students better at problem-solving questions should take the SAT and students better at content-based questions should take the ACT.
However, with the redesign of the SAT in March of 2016, both the SAT and ACT are more similar than ever. Therefore, it does not matter which exam your student takes. Colleges and universities do not prefer one exam over the other. One advantage of preparing for the SAT is that it will also prepare students for the PSAT that is administered in October of their junior year, which can qualify them for National Merit scholarships!