Honestly, its a matter of your personal tastes. I only took the SAT because it is much easier to find a testing center for it in my country than it is to find an ACT testing center (I'm international) The SAT consists of 3 sections Critical Reading, Writing, and Math. It involves an essay portion as well. I've heard that the essays are generally more difficult than the ACT topic-wise. Generally I've seen friends who are more humanities-based as in writing, social sciences, etc. do well on the exam with little to no prep. ie. Friend who got 2330 very little prep. I, am more science and math based and had to prep a little more to get the same score.
Thats my two cents on the SAT at least.