A metric socket set will generally include a wide range of metric socket sizes, a driver, and a container for holding all of the pieces. Some sets will even include an extender or adapter for the sockets, though not all sets will include this piece. Some sets will even include more than one driver. When choosing the best metric socket set for you, be sure to consider how you are most likely to use the set. This will often dictate the size of the set as well as the size of the driver.
Consider your budget when going shopping for a metric socket set. Larger sets can be quite expensive, but they will include all the sizes necessary to work on a wide variety of jobs. Such sets often include more than one driver of different sizes, and the sets may also include a swivel driver, which is a good investment for anyone who will be working on automobiles. The swivel driver will allow you to access bolts in odd positions by swiveling into positions not possible with a rigid driver. Consider this extra investment if you intend to use the metric socket set for a wide variety of jobs as well.
Think about what size driver you will need. Larger drivers are good for applying torque to tight bolts, turning larger bolts, and being flexible enough to handle a wide variety of jobs. Larger drivers do, however, tend to be somewhat clunky in tight spots, sometimes making it impossible to actually turn the driver. Extenders are useful for overcoming this problem, but the extender may not be sufficient to allow room for the driver to turn. Smaller drivers are great for smaller jobs, especially jobs in tight spots in which turning room is very limited. Smaller drivers and sockets also tend to be less expensive.
Be sure to do a bit of research to find out if the manufacturer of the metric socket set includes any type of warranty at the time of purchase. Many high-end tool makers will include a lifetime warranty against manufacturer defects, so if the drivers or sockets break suddenly, there is a good chance the manufacturer will replace the unit free of charge. Others offer a more limited warranty, so be sure to look into what is included with the metric socket set you are considering. Some stores will offer a return policy as well, so be sure to find out what options you have for returning defective goods or a set that does not fit your needs.