Do selective colleges favor the SAT over the ACT?

It seems to me that the SAT is a more difficult test overall, so I find it hard to believe that colleges look upon the SAT and the ACT as equal measurements of academic potential. Is there a subtle bias towards applicants with high SAT scores over those with high ACT scores at elite colleges? If not, why does anyone take the SAT when the ACT is overall an easier test?

There is no solid evidence to suggest that elite colleges favor the SAT over the ACT. The idea that highly selective colleges favor the SAT may come from the fact that a higher percentage of admitted students at Ivy League schools take the SAT than the ACT. This may seem like an indication that these schools favor students with high SAT scores, but it's actually just a result of regional application patterns. The SAT is more common than the ACT in the Northeast, and more people from this area of the country apply to Ivy League schools. For more details on why the ACT is viewed as equal to the SAT, read this article.

To answer the second part of your question, some students feel that one test is easier than the other, but there isn't an objective consensus on the relative difficulty levels of the two exams. You may think the ACT is a much easier test, in which case it would make complete sense for you to take it instead of the SAT. However, this isn't true for all students. The ACT involves more time pressure and an arguably more challenging Math section. Here's an article on this topic as well if you're interested in reading more.