if you plan on planting anything and are deciding between plain gravel or sand, i would say sand. i think sand looks a lot more natural for most tank setups and its very appealing. it is also better for plants (unless were talking a made-for-plant substrate) since it doesn't hinder the roots as much. as far as the substrate holding BB, it does play a role, but i would say the difference between sand and gravel for holding BB is really insignificant.