There are many factors to consider before deciding which test to take.  On one hand, the answer is quite simple; both the SAT and ACT are standardized tests recognized by virtually every US college and thus it really doesn’t matter which you take.  But the question is usually more complicated than that.  Most students want to know which test is easier, or which test is preferred at MyNumberOne College, or which test will get me more scholarships.

As a general rule, the best answers to any college admissions question will come straight from the source: the college admissions committee.  Thus, if you want to know which SAT Subject Tests are most frequently taken by students admitted to Harvard, call up Harvard’s admissions department and ask.  They usually compile statistics on such things and helping you is what they’re paid to do.  So to find out which test you should take, you probably should know which universities you are interested in.  If you don’t know, compile a list of ten things which you want to get from your college education.  Area of study should probably be high on this list (if you’re undecided, you’re going to want to pick a school which keeps your options open).  Do you want to go to a ginormous school where you’ll network well and watch the best football team, but have professors who will rarely know your name?  Or would you prefer the small liberal arts college where you’ll feel part of the community and take classes taught nowhere else?  I digress - but to know which test is in your best interest, you should probably know which schools you want to go to and call them to find out what tests they prefer.

From my own research, the SAT is a test which has traditionally been associated with colleges from the East Coast (including the Ivy Leagues) and the West Coast.  The ACT was a Midwest and Southern test.  But now most schools accept both, so the geographical boundaries are more or less irrelevant.