There is no “one size fits all” answer. But here are some general guidelines.
Two years ago, the advantage went to taking the SAT because 95% of colleges superscore the SAT, and, at the time, hardly any colleges superscored the ACT. Today, still only about 40% of colleges superscore the ACT, so, in this respect, taking the SAT still has an advantage.
When the SAT changed last year, however, the overall advantage switched to taking the ACT because of the comparative lack of official content for preparing for the SAT. This difference between the two tests, because it has nothing to do with the content of the tests or how they are used by colleges, is often overlooked, but it is more impactful than any other difference. Only 7 real SATs have been released; compare that to over 50 real ACTs. Let me explain why that difference matters so much.
The SAT and ACT spend about $300,000 developing each real test. Third party providers of tests spend, I can only guess, maybe 2% of that same budget developing each 
of their practice tests, and these third party tests vary dramatically in quality. The Princeton Review practice tests are comparatively good; the Barron’s tests (though I like their books for other subjects) are almost complete rubbish. Yet even Princeton Review’s tests pale in comparison to the quality and accuracy of the real tests. There is simply no substitute.
Because of this limitation, students preparing for the SAT and trying to substantially increase their score encounter the real problem of running out of authentic content on which to practice because more real content simply does not exist. When this happens, they can only rely on practice tests that are significantly different from the real tests and do not give accurate estimations of what scores students will receive when they take the real tests. The lack of accuracy in practice scores is almost as bad as the lack of accuracy in the content because it does not allow students to assess or track their progress.
The abundance of real content for the ACT allows students to more easily and efficiently continue raising their scores beyond a comparative increase on the SAT. Thus, while it is still important to assess which test is best for each student by having them take a mock test of each, the advantage for most students goes to taking the ACT. Only if a student does dramatically better on the SAT (which sometimes happens due to the differences in the tests) is taking the SAT equally or more advantageous. Otherwise, in the marathon for a large score increase, the ACT wins.
