Most colleges accept either the ACT or the SAT, and neither is inherently "better" -- but the two tests are structured differently enough that one may genuinely suit your strengths more than the other. Here's an honest, current comparison.
Format and timing
The ACT's three required sections -- English, Mathematics, and Reading -- run 125 minutes for 131 questions, with an optional Science section and optional Writing essay available on top of that. The digital SAT, by contrast, has just two sections -- Reading and Writing, and Math -- split into four adaptive modules, running 2 hours and 14 minutes for 98 questions total. The SAT is notably more time-per-question than the ACT across the board.
Related: What to Expect on the ACT: Format, Scoring, and How to Prepare
Adaptive vs. fixed difficulty
This is one of the biggest structural differences. The SAT is adaptive: how you perform on the first module of each section determines whether your second module is easier or harder, which affects your final score. The ACT has no adaptive component at all -- every test-taker sees a fixed set of questions of mixed difficulty in a set order, regardless of how earlier questions went.
Calculator policy
The ACT permits a calculator for the entire Math section, but with restrictions on certain calculator features (like built-in computer algebra systems). The SAT's digital format provides a built-in graphing calculator (Desmos) available for every math question, with no restriction on bringing your own approved calculator as well.
Science section
The ACT has an optional Science section testing data interpretation, graph reading, and experimental reasoning -- not a test of memorized science content, but of extracting and reasoning about scientific data. The SAT has no Science section at all, in any form.
Reading and Writing structure
The ACT keeps Reading and English (grammar/usage) as separate sections with distinct question formats. The SAT's digital format combines Reading and Writing into a single section built from many short passages (one or a few questions each) rather than the ACT's longer, multi-question passages -- a real structural difference in how you'll need to manage your attention and pacing.
Which one should you take?
Both tests are accepted essentially everywhere, so the honest answer is: it depends on your own strengths. If you prefer working through longer reading passages with multiple related questions, and don't mind an optional data-heavy Science section, the ACT's format may suit you better. If you prefer shorter, more varied passages and want a calculator available for every math question, the SAT's format may feel more comfortable. Many students take a full official practice test of each before committing to one -- it's a more reliable signal than guessing based on reputation alone.