I’d suggest that students take both. There is some scale which relates ACT scores to SAT scores, which I remember looking last year (Note: I’m a high school student). All I remember is that a 33 corresponds to about a score around 1450, and I managed to get a 1480 on the SAT.
The ACT, as you know, tests particular subjects: English, Reading, Math, and Scientific Reasoning. The SAT, on the other hand, has only two sections, Verbal and Math. Originally, the SAT’s purpose was to determine how well a student would perform in her freshman year of college. Obviously, we know that the SAT, nor any other standardized written test, can actually accurately test something like this.
There’s also the SAT II: Subject tests, which many colleges nowadays require.
To the poster who replied that one should take the SAT if applying to Ivy League Schools – I don’t think that’s necessarily the case. I remember many applications stating that one could submit only ACT scores, if the student had only taken the ACT. If both the SAT and ACT were taken, then SAT scores had to be submitted as well.
Anyway, my scores helped me get into the school I wanted. But test scores are one of the least things private schools look for. In the end, it’s really a crapshoot.
At any rate, I think standardized tests, namely the SAT and ACT, are a bunch of crap.