Small chips of a medium stay more moist than large chunks of the same medium. These media dry at different rates. The first ones listed stay the most moist, the latter tend to stay drier. If you're prone to overwater, think about using the drier media. If you prefer to water less, use the moister media.
Many people combine media to suit their watering habits and preferences.
No plant will live on air and water alone. Most orchid potting media provide support only, and have little or no food value. Use a water soluble fertilizer at the dilutions recommended on the label. Plants in lower light need less fertilizer or the tips of leaves start to shrivel. Flush the pots with plain water occasionally to prevent deadly salt buildup.