Sand is a pain in butt to clean, dont care what everone says, tried it (many times) cant stand it. Gravel is preferable, I like the very small fine gravel.
Against sand: It gets into HOB filters and chews up the impellers. It won't stay in one place, but tends to pile up on itself while leaving bare spots of glass bottom in others.
I also use dark fine gravel, don't no why just like it i guess. I would like sand it does look nicer, but i think in the long run - im better off with gravel.