White sand is cooler because it reflects nearly all of the sun's rays. Black sand absorbs nearly all of the sun's rays. It's the same reason that when you park your car in the sun, if it is a light color, the fender won't feel as hot to the touch as if it is a dark color.
One of the most interesting beaches I remember was Muriwai Beach in New Zealand. Black sand, and a very flat beach. As you looked out toward the ocean, it looked like the water was higher than the land. I'm not sure if the optical illusion was because of the flatness of the beach, or the black sand, or a combination of both.
I like a white sand beach better, though. But be careful. Because of the reflectiveness, you can get a sunburn more quickly on a white sand beach.