Choosing the best aspect ratio can be difficult. Using 4:3 aspect ratio uses the full sensor on your iPhone. The other options crop in from the full sensor and the benefit is that you don't have to crop it in post production. If you want to simplify your workflow; setting an aspect ratio to desired outcome of either square or 16:9 can be useful. If you are like me, I prefer to capture the widest aspect ratio; then crop in post production so that I don't accidentally miss the shot.