With a very few exceptions, US universities accept both these tests and treat them the same. In their current formats, the ACT is broader than the SAT, covering more ground with less depth, and will be more in tune with your own education. That said, the SAT is the more common test for international students to take (probably through the broader selection of testing centres).
How they are similar
There are many similarities between the ACT and the SAT: both tests are standardised around the globe, largely multiple-choice format, offered on Saturday mornings throughout the year, and are about 4 hours long. Also, both accommodate disabilities and can be re-taken multiple times.
Now that the new SAT is in place, there are additional similarities with the ACT – there is no penalty for wrong answers and the essay component is optional (previously this was only true for the ACT).
...And how they're different
Even after the new SAT up and running, there will continue to be some key differences between the ACT and the SAT: the topics covered (the ACT adds Science Reasoning), the scoring range and method, and the greater availability of test centres for the SAT.
How to choose?
If you’re trying to decide between the SAT and the ACT, you should try a test for each and see which one of the two you are most comfortable with. Sample tests and practice questions are available on SAT website and ACT website.
There is one more consideration that might help you make that decision, but this is dependent on the universities you are considering.
In most instances, any requirement to sit SAT Subject Tests will apply regardless of whether you have taken the SAT Reasoning Test or ACT. However, because the ACT is more content based than the SAT, some US universities that have Subject Test requirements will fully or partially waive this requirement for students who take the full ACT (including the Writing section). For example, both Yale and Columbia fully waive subject tests if the student takes the ACT; Harvard does not.
(Caveat about the above: unis change admissions requirements all the time, so this information could change by the time you read it. Check and recheck requirements for the unis you’re interested in).