Materials matter. Generally hard bags are made of polycarbonate or ABS (Acrylonitrile-Butadiene-Styrene) plastic. Both are tough, strong, and light, so the main difference really boils down to what kind of abuse they can take. Polycarbonate is generally more flexible, so it’s better able to take impacts, but ABS plastic is generally lighter and cheaper. Both can handle your typical baggage handler with ease, but consider polycarbonate if you’re roughing it.