Which job interests you more? Do you already have teaching and research experience from your other activities?
Agreed with Moko. Is your app deficit in either space? If you have teaching experience but no research, I'd say research assistant may benefit your app more. Or vice versa. If you have little experience in either, I think high school teacher would stand out more imo. You also need to take into account which you'd prefer to do. Both would look good on an app.
From an admissions standpoint, there are diminishing returns with any activity. If your research experience is objectively weak and you plan on applying to schools with a strong focus on research, it may make sense to have more research experience, especially if the research assistant job is different from prior experiences. Personally, I hated bench research, but found clinical research to be fun. So it's helpful to explore different types of research.
Your ECs should tell the story of who you are. Are you a researcher or a teacher? Where is your passion? What would you be excited to talk about during an interview.
I think you've answered the question. Don't get caught up in the rat race of SDN.
