First the cost is relatively the same....check out the sites for each test because it depends on timing (early or late) and the tests (subject, with writing, w/o writing, etc)
Also, confirm that the institution you are applying to accepts both exams and that will help you decide which you shoud take
Differences: ACT has a science section and is often suggested for students who are strong in the science field; additionally unlike the SAT the ACT is set up so that when you complete Math, you do not return to it, The ACT has critical reading, math and science sections
SAT focuses on Critical Reading and Math only and is set up so that you may start with math, move to a reading section, return to a math section, go back to another reading section and so on, for some students it is more frustrating but if you are well versed in the areas you are fine with the structure. The SAT offers subject area tests that can be used to place out of a general education requirement.
Both tests have a writing section that most colleges require.
Basically, I advise students to take the ACT if science is a very strong area for them, it can often increase their score comparison and may be an advantage over the SAT alone, I do believe both tests provide an opportuntiy for some students to showcase their ability to tests well on prior gained knowledge, but it is not an absolute indicator for all students. Those who do not tests well may be slighted on either tests, not because they do not know the information, but the test anxiety is so high that they freeze and do less then they are capable of.
Test prep courses are expensive and have not been shown to boost scores high enough to out way the cost; reading and practicing using the many self guided resources available in books and on the internet may provide just as much opportunity as enrolling in a course and save your money to take the tests several times to get familiar with it and cut down the anxiety level by knowing what to expect.