My DD actually did both tests. This was beneficial in that it exposed her to both tests and also allowed us to see the differences in the tests. The interesting thing was that she qualified for awards in every area except math on the ACT and on the SAT only qualified with her math score. Obviously we will keep this in mind when it is time for her to take the tests in high school.
If you just want to expose a child to a test and do not care about qualifying for something, it is cheaper and easier to just have her take one of the released tests (they are available for both the SAT and ACT) at home.
The ACT math section covers more math, such as trigonometry, than the SAT math section, so your daughter's relative underperformance on the ACT math may simply indicate unfamiliarity with some tested concepts.