Both have added value. SGA or any extra curricular activity will diversify your resume and will show that you are an active member in your high school community. It will teach you life skills, such as time management, that cannot necessarily be learned in a classroom. However, if you're already very involved in your school, I would recommend taking an AP class. It will better prepare you for the study time needed in college classes. And assuming you pass the exam, you will probably get college credit for the class. If you know where you are going, check to see if the AP class will count for college credit so you don't have to take the class again.
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AP classes are typically structured to provide the academic rigor comparable to college courses. Great preparation for college. Also, when students pass the AP test they receive college credits. Working at universities for over 17 years I have seen the many benefits for students.
If you are preparing for the college admissions process, AP courses have more value. Leadership training can be pursued as an extracurricular activity, which you can document on applications and discuss in essays or interviews. Joining Toastmasters International is a great, low investment high return professional development and social vehicle. It combines communication and leadership training in well-developed programs with certifications. Good luck!
While leadership classes are important for your future, AP classes give you more of a taste of what college is like. You will see the different level of studying required, the types of requirements that will be requested of you during your college career, and if you pass the AP test, you can receive college credits.