1) ISO 400
The correctly exposed image is clearly the best one, because it keeps the white background white and doesn't overexpose his skin or hair, and also doesn't darken the image to the point where the flaws in the white backdrop can be seen.
2) ISO 1600
The noise is worse in the underexposed image.
3) RAW vs JPEG
The JPEG image is quite pixelated at 100%, whereas the RAW image is not.