One is not better than the other. Each has its own pros and cons, so that’s why it’s important to do both.
High school ball is more organized and allows college coaches to get a more comprehensive analysis of your skills. The downside is that it is during the same time as college season, so coaches do not have time to come see you play live.
AAU ball is better for exposure because it is played during the college offseason. This allows more college coaches to come watch your games in person. These opportunities are key for relationship building with the coaching staff.