Hey, sorry, but both looks are one night standish on anyoone over about 16. Might be cute on someone really young, but not for a WOMAN, it's too revealing; tight skirts like this are more revealing than a bikini in some ways, because they ENHANCE curves so much. If you're going with a short tight skirt, better tone it down, flat shoes instead of heels, matte colors, or guys are going to think she's advertising her availability, and HOW!
I'm gonna be totally honest and say B, though I don't think that's the answer you want!
When I'm looking for more than a one night stand, the clothes that a woman is wearing on that particular day are definitely not topping my check-list. Clothes will change. Her behavior is ALWAYS the key factor.
Why would I prefer B then? It would get my attention, show some degree of confidence and (importantly) immediately attract me from an aesthetic perspective. IMHO, there isn't that much more that clothes should do.
How "Sexy" somebody is depends on more than clothes.
I.e. you could wear outfit A, and not be at all sexy if you hurl yourself at every guy around/get wasted/act disrespectfully.