Regarding the difficulty of the SAT/ACT, each test measures something different – the SAT measures reasoning (requires the use of fluid intelligence), while the ACT measures your knowledge of certain subjects (tests more crystallized intelligence).
Although some people may do better on the ACT, others do not, including me. I found the ACT much more difficult in time management. There is also a science section on the ACT, which has been beneficial to many test-takers that excel in science.
I found the SAT very easy to improve on; after receiving my first scores, I studied for the writing section one-to-two weeks before my next test and raised my score 100+ points. The math section is also easy to improve on if you use the right materials. The reading section is definitely the hardest to improve on since it requires years of practice. However, there are some guides that can teach you some helpful tips (these are not guaranteed to help you though, and can potentially hurt your score - like they did to my score!).
Keep in mind that the collegeboard recently created a new SAT that has new criteria and different score penalties. As the previous commenters have mentioned, I would take practice tests for each to see which you are better at.
Good luck, and if you have already taken the SAT/ACT, then good job at getting that obstacle out of the way!
If you still need to take it, then do not stress out! Most kids have either not taken their test(s) yet, or are unsatisfied by their scores, and plan on retaking them in the fall.