Before you can choose which test to take, you need to understand how they're different; then you can pick the one that plays to your strengths (or avoids your weaknesses):

• Length: The ACT can be almost a full hour shorter than the SAT (2 hours 55 minutes versus 3 hours 45 minutes) if you choose not to take the ACT's optional half-hour Writing section. If taking an exam stresses you out, you might choose the ACT just to get through it more quickly.
• Experimental sections: Both tests can include an experimental test section: in the ACT, that section is clearly labeled; in the SAT, it is included as one of the regular sections, so you won't know which one it is.
• Science: The ACT includes a science section that tests your ability to analysize and interpret data; the SAT has no such section. If you're a science wiz, the ACT gives you the opportunity to strut your stuff.
• Math: Both tests cover basic math, algebra I and II, and geometry. The ACT, however, also includes questions that involve trigonometry. If you've taken those upper-level math and science courses, you can show it on the ACT. But if math is one of your weak points, you might be better off sticking with the SAT.
• Reading comprehension: Both tests have reading comprehension sections. You'll need good reading comprehension and analysis skills regardless of which test you choose.
• Essay writing: Both tests have an essay-writing section, but it's optional for the ACT. If you know your writing isn't up to par, the ACT can be your friend. (However, some colleges require that you take the Writing section; check with the admissions office of your target colleges before you opt out of it.) The Writing section is the last thing you'll do on the ACT, but the first thing you'll do on the SAT.
• Scoring: The ACT results in a composite score between 1 and 36; the SAT gives you a total score between 600 and 2400. On the SAT, you are penalized a quarter of a point for wrong answers; the ACT has no penalty for wrong answers.

Some people choose to take both tests and then send the better score on to colleges. This is certainly a viable option, but you're setting yourself up for a lot of stress preparing for both tests. And your scores aren't likely to be wildly different (relatively speaking) between tests anyway.