Mikey is right, its a matter of preference.
I like sand, its cheaper than gravel, it keeps the tank cleaner, and the dirt sits on top where you can see it - so you can remove it easily. And finally, as fluttermoth points out, sand is ideal for certain bottom dwelling fish. In my opinion, cories take on a whole new behavior in sand as compared to gravel. I'm a fan of watching the cories sift through the sand and push it through their gills as they do so. They really do seem far "happier" on sand. I'd never go back to gravel with cories, but again, that's personal preference more than anything else. Proper, rounded gravel is just fine for cories, I think.