Several studies have tested why certain men are considered more "masculine" and it has been shown that features rated as masculine (such as square jaws) are a result of high testosterone levels. However, testosterone suppressed the immune system, so men with high level of testosterone (the rough-looking guys) are more vulnerable to infection.
So some women might have a natural preference for guys who will live longer (with lower level of testosterone), while others (most of them perhaps) are more attracted to the rough guys they are more likely to ahve children with.

some have said that it depends on the "time of the month" (when you're due your period, your hormones shout for the tough guy) which, in turn is linked to survival and who looks more likely to spawn healthy offspring. the freudians out there will say that women go for blokes who subconsciously remind them of their fathers (mine looks remarkably like my brother who looks like my grandfather did at that age).