Picking the substrate for your aquarium might not seem like a big deal, but it has serious consequences for your aquarium. Gravel and sand are common substrates, and each has pros and cons. Each is right for certain aquariums.
In most cases, pea-size gravel makes the better substrate for freshwater aquariums. Pea gravel is large enough that it allows water to flow through it, preventing anaerobic "dead zones" where harmful bacteria can thrive and produce toxic compounds. At the same time, it's not so large that it creates gaps where fish food can fall beyond the reach of the fish and rot. On top of this, it's too large to get stirred up easily and sucked into filters. Pea gravel comes in a wide variety of colors.
Despite the many benefits of gravel, sand work very well in certain setups. If you have species that like to burrow or sift through the substrate, aquarium gravel can make life difficult for them. For example, certain small cichlids and loaches love to burrow. In the absence of burrowing fish, sand can lead to anaerobic dead zones. Sand works great as long as you have creatures in the tank that will burrow through it or otherwise stir the sand.
Planted tanks are a special circumstance. In a planted tank, you have to consider the substrate preferences of your plant. Some aquarium plants thrive in sandy substrates, while others just need something to stick their roots in. Research the plants you want to keep. You can add fertilizers in tablet or stick form to both sand and gravel substrates. You can also buy special, pre-fertilized substrates just for planted aquariums.
With gravel or sand, you will have to decide how much to get. The recommendation is 2 inches of gravel for tanks up to 55 gallons, and 3 to 4 inches for larger aquariums. With sand, you want to have 1 inch of sand for tanks with small burrowing fish and 2 inches for larger burrowing fish. A good guideline for most tanks is to purchase 1.5 pounds of substrate per gallon. However, this guideline is inaccurate for tall or unusually shaped aquariums. Resort to trial and error if necessarily.