When selecting a voltage regulator, there are three primary questions to ask: what is the input voltage range, what is the output voltage required, and what is the output current required? These will determine if a switching or linear regulator is needed. If there is a significant amount of voltage to drop, e.g. converting 30V to 5V, than a switching regulator would be the best choice. If a smaller conversion is needed, e.g. 5V to 3.3, than an LDO might be the best solution. In many cases the answer will depend upon space, cost, efficiency, and environmental (mostly thermal) restrictions.