Generally, the SAT Subject Tests are taken as a junior and the results are included in the packet that is submitted to colleges as part of the application process. In fact, many top colleges require a minimum of two-three relatively high-scoring SAT Subject Tests (usually math, English and one other) as part of their application.  Most colleges and universities publish their admission requirements explicitly when it comes to standardized testing, so it should be the duty of all parents and students to check the requirements of each targeted school well in advance.

In general, AP tests have nothing to do with getting into college. AP exams are taken most often as a senior, and as such, the results will not be part of the college application for many students.  AP tests are used primarily for gaining college credit or "placing out of" the first year or introductory-level college courses.

There are significant advantages to simultaneous SAT Subject/AP testing as a junior.  In short, it's a read once/test many strategy.  If you can qualify for one or more AP classes as a junior (or complete a successful independent study program with the help of an Campbell Tutoring tutor), and score a B or higher, you can take the SAT Subject Tests and AP tests late in the school year when all that knowledge is fresh in your mind.

The SAT Subject Tests are considered somewhat easier than its corresponding AP test.  SAT Subject Tests are multiple choice, whereas the AP test usually involves both multiple choice and some essay writing.   SAT Subject Tests are said to be more knowledge-based or fact-based, while the AP test involves more discretion and analysis.