I thought about doing both, but I don't want to waste time and money if I should be focusing more on one. What do you recommend?
AP courses and exams are more valuable to colleges than SAT Subject Tests overall. AP exams are more challenging than SAT Subject Tests, and they have more of an impact on your admissions chances and ultimate college course load.
Your grades in AP courses will contribute to your GPA, which is the most important factor considered by colleges when evaluating applications. Additionally, your scores on AP tests may qualify you for higher level courses in college or even allow you to test out of certain required classes.
SAT Subject Tests, on the other hand, are largely unnecessary for most students. You can find a list of all the schools that ask for them here. You'll notice that it's a pretty small subset of colleges, and most don't list Subject Tests as actual requirements for admission. Subject Tests are more of an add-on for your application that will showcase your skills in a specific academic area that you want to emphasize. Also, since they're easier than AP tests, students often take them after AP classes in the corresponding subjects without any extra studying.
My advice is to prioritize APs, and put Subject Tests on the back burner. Check out the admissions sites for schools that interest you and see what their policies on Subject Tests are. If you notice that they're recommended or required, you can think some more about which Subject Tests fit your skills and goals best. In the meantime, you can find a more detailed comparison between AP exams and Subject Tests in this blog article.