Should you take the SAT or the ACT?
Colleges accept both equally, so the choice is about the student, not the school. The ACT is faster with a science reasoning section; the SAT gives more time per question and is digital and adaptive. Take a full timed practice test of each and let the scores and the experience decide.
The real differences
The ACT runs at a quicker pace and includes a science section that is mostly about reading charts and experiments rather than recalled science knowledge. The SAT allows more time per question and is now delivered digitally in an adaptive format, so the second module adjusts to performance on the first. Students who read quickly under pressure often prefer the ACT. Students who need thinking time often prefer the SAT.
How to decide
Take one full, timed practice test of each under real conditions, then compare using an official concordance rather than raw scores, since the scales differ. If one is clearly stronger, commit to it. Splitting preparation across both is the most common wasted effort in test prep.
How much preparation is reasonable
Familiarity with the format is worth real points and costs little. Beyond that, returns diminish and the cost, in money and in a junior year that is already full, climbs quickly. A student sacrificing grades for test prep is usually making a bad trade, because the transcript carries more weight than the score at nearly every school.
Remember what a score is for
With many schools test optional, a score is now a tool you deploy where it helps rather than a universal requirement. Decide school by school whether to send it.