Neither the SAT nor the ACT is “easier” or “harder” than the other – but different types of students usually do MUCH better on one than they do on the other.
I tutor both the SAT and the ACT and parents usually come to me with a good idea of which test they/their students want to prepare for and only the minority even consider taking both.
This can be problematic because some students will do far better on one exam than the other.
Thus what I advocate for most of my students is to take one practice test of each at the outset of their preparation (unless of course, you are already decided on which test you should take). This is easily done. Believe it or not, you do not need to take a practice test in a proctored environment offered by a test prep company only to be given your results after hearing a sales pitch. Timing yourself is all you need to do; anyone can interpret their results given the right information.
So what are the differences between the ACT and SAT?
1. The SAT tests far more vocabulary than the ACT does.
The SAT has questions that specifically test vocabulary. Each SAT has 19 questions over the three Critical Reading sections that test vocabulary almost exclusively (along with comprehending the sentence context). These are sentence completion questions; you have to fill in the blank from a bank of multiple choice answers. Additionally, there are some vocabulary questions within the actual reading comprehension passages.
Having a strong vocabulary obviously will help students on the reading and English portions of both exams. However, if you're child has a very weak vocabulary, the ACT will pose far less of a challenge than the SAT. On the other hand, a strong vocabulary will be of a better benefit to you on the SAT.
2. The ACT has slightly more complex math, covering a much broader array of topics
Both exams require strong knowledge of arithmetic, algebra, and geometry. The ACT requires more than that though, covering topics such as trigonometry, matrices, imaginary numbers, 3d spatial reasoning, logarithms, quadratic equations and more.
However, the ACT math questions are definitely more straightforward while the SAT questions are far more "tricky". Thus I don't believe that either section is more difficult than the other.
Realize, though, that if you have not studied these more advanced materials, you might be better off on the SAT.
3. The ACT has a "Science" section that has almost nothing to do with science.
A lot of students are horrified by the prospect of the science section...until they actually start learning about the test and how it's composed. In reality, the section should be called "the how to use common sense" section. There is no such parallel on the SAT.
4. The SAT is intentionally confusing but easier, while the ACT is harder but far more straightforward.
Take Math, as discussed above as an example. The SAT math questions require less knowledge and, when you can figure out what the test is asking and figure out what they're doing to confuse you, are easier to answer. The English and Reading sections are essentially the same way. This effect is compounded when you consider the timing differences between the two: the ACT is fare more time rigorous and requires you to focus for much longer periods. The SAT breaks up the sections into smaller increments, so while there are more sections, they are all shorter and require less focus that way.
So, do colleges prefer one test over the other?
Simply put, no.
Okay, a bit more information. The SAT and ACT can be matched up identically. Colleges just prefer higher scores to lower scores. That's it. A 36 on the ACT is identical to a 1600 on the SAT. A 21 on the ACT is identical to a 1050 on the ACT. And so on and so forth. Simply take the test you're better at taking and all will be well.
Okay, so what now?
Take two timed practice exams, one of the SAT and one of the ACT. Do whichever one you like better or get a better score on. If you have absolutely no preference and perform about the same on both, follow your gut or flip a coin. Better to spend your time prepping for either than to spend it on deciding on which to take.
Of course, you can contact me for a free recent ACT or SAT and all the information you need to interpret your results.