Personally, I found the ACT easier than the SAT practice tests I had taken as I LOVE reading and strongly dislike math. The ACT tends to be easier to get high scores on due to your composite score being an average vs. a sum like the SAT. However, as @happy1 recommends, take a few practice tests and choose whichever one you like more.
Some free official practice tests can be found here:
- Khan Academy SAT Practice: https://www.khanacademy.org/sat
- ACT Academy: https://www.act.org/content/act/en/products-and-services/the-act/test-preparation/act-academy.html