Should I take the SAT subject test if I am already taking the AP test in that subject?
It depends. Some AP classes will not cover the breadth of material that is covered on a Subject Test; others will. So you might need to put in some work to self study the gaps.
“Better” is relative. They’re different. And except for foreign languages where some colleges might use either to exempt a student from a foreign language requirement, one does not take the place of another. Subject Tests are used for admissions purposes, while AP exams are used for potential credit and/or placement.
If you are going to apply to schools that require or recommend SAT2 scores, then taking the test on the same subject of AP would make sense. Even if they do not cover the exact material, it would not be much more work to get prepared for it.
SAT Subject Tests really depends. They may cover the same material but the format is usually different, and sometimes the subject tests are harder. That being said, it is probably a good idea; but you should check. For example, AP Physics 1 does not cover all the material in the SAT Subject Test Physics.
To give another scenario similarily to Alumon, AP chemistry covers everything on the SAT subject test except organic chem. The organic chem for the subject test is pretty basic(just memorization). If you look towards history, we all know that history does not change. This means that AP US&AP WH cover 100% the same material as the SAT2 for those fields.