I have black sand (3M Colorquartz sand) and it's awesome. I used to have brown gravel, but if I got behind on waterchanges, all the fish poop and rotten food would go into the cracks of the gravel and stay there. With sand, especially if you have a pretty good current, most of the poop will stay on top and eventually make its way to one of your filters to be picked up.
Also, the type of fish I have makes sand an easy choice. I have geos and bichirs. The geos love to dig for food in the sand and spit it out, and my bichirs search for food on the sand and I don't have to worry about a big piece of gravel being stuck in their throats. Plus, black sand/black background really makes geos and bichirs color up nicely, much better than gravel IMO.