ISO

400

5000

ISO is the speed that the light comes into the camera. The higher the number the more grainy and bright the photo will be. The lower the number the better the quality will be but the photo will be darker. Over the course of the pictures they gradually get lighter the higher the ISO.
Flash creates bad lighting and ugly backgrounds and ISO don't that's why i would choose it over flash. You have to be careful about it coming out grainy.