<p>Most people do best on one or the other. Most schools (almost anywhere most people want to go) accept ACT scores. Several schools exempt you from SAT II tests if you do the ACT with writing. An 1800 is an above average score, and with studying you could increase your score significantly, but if I were you I would just focus on the ACT because you can even improve your score there. No worries! Lots of people only take one test.</p>