It is not just a matter of look. If you are buying an 100% cotton or an 100% wool sock, the only stretch part of the sock is the top, where you usually have spandex to hold the sock to the leg. In the body of the sock you have no nylon and no spandex. The rib is giving more fitting to the sock, as the reverse points (the classic "derby" rib is 7 points on the front and 3 reverse points) are giving more flexibility to the knitting. So if you would like to have an "extra" fitting in a 100% natural fiber sock, better to go for the ribbed one, unless you prefer the flat knit for "fashion" reasons.