ISO

ISO 5000

ISO 800

ISO 400

FLASH

ISO, ISO is how fast the light gets process in the camera. in other word, it means that the more or less light you going to allow going in your picture persay. then you used flash, you kind of freeze the moment and it scared your subject. also, it darken the brackground, while in ISO, you can allow how much light into the picture and how fast the picture flow is going to be. i would pick ISO over flashing because flashing gives my emphasis red eyes and it produce an ugly lighting and darken the background so i do not want that happen in my picture. you have to hold the camera still and your emphasis have to stay still the whole time while the picture is taken because one slightly movement will blur out your picture.