I took both (new version of the SAT). I preferred the ACT for the following reasons:
* ACT emphasizes grammar and spelling, rather than reading comprehension. Reading comprehension questions can be very tricky to discern between the right and wrong answers, whereas grammar is pretty much straightforward with a few exceptions.
* Math sections on both were fairly easy, but the ACT tested on concepts that were more recently learned.
* The ACT had science and history sections, whereas the SAT only had the English (both multiple choice and essay) and math. Since I enjoy and am good at science and history (more so than English), this was more of an advantage.
I found that the essay sections in both were fairly comprable. They were very different questions with similar ideas and I got the same score on the essay from the SAT and the essay from the ACT.
If you're thinking about taking either (especially the SAT) and especially if you are concerned about nerves, I would advise taking a practice test or two. Get a book with old exam questions from the store instead of going to a class - unless you really aren't comfortable with taking multiple choice exams. That way, you can spend a little more time on what you need to work on.
Also, the ACT does not penalize for guessing, while the SAT does. Usually, the rule of thumb is that if you can eliminate one or more of the wrong answers, you should go ahead and guess. However, this varies from person to person. I knew someone in my AP chemistry class who could narrow the multiple choice questions down to two answers (one being the correct one) and this person would consistently guess wrong.
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Itaque incipet.
All that glitters is not gold but at least it contains free electrons.