I pair the crop top with the black pencil skirt. The crop top is too revealing and shouldn't be paired with a short skirt or anything with a low waist. The tight black pencil skirt doesn't reveal so much skin, it is high waisted, but it's sexy because it is tight and clings to your body. That combination makes you sexy in a good way, while the other one will make you look slutty.
Crop tops go best with knee length or longer skirts so I would say the black. Short top with short skirt I think is just a lot of short. I actually think the black skirt is sexier. It's tight, mysterious but find of refined because its not too short.