The type of strings installed on an acoustic guitar has much to do with its playability and sound. Although thicker medium acoustic guitar strings tend to sound louder, they can be much more difficult to play than light strings. Medium strings may also put undue pressure on an acoustic guitar's neck and bridge, sometimes resulting in expensive repairs.
Acoustic guitar strings are sized by gauge--the higher the number, the thicker the string--and are sold in sets. Gauged sets are calculated by string manufacturers to produce even tension across all areas of the neck.
Typical medium sets from high to low: .013-.017-.026-.036-.046-.056. Typical light sets from high to low: .011-.015-.022-.030-.042-.052.
Medium strings produce more tension on delicate guitar parts and require the player to press down harder when playing. Playing difficulty can be compounded when an instrument is out of adjustment, causing the strings to be farther away from the fretboard than is necessary. Light strings produce less tension and are easier on the guitar and player.
Sound is the real reason some players choose medium strings. Thicker strings project more volume and provide a more "solid" sound due to the metal strings' mass. Light strings project less volume and provide a "looser" sound. Players who choose lighter strings have weighed the pros and cons and choose playability over volume and tone. Light strings sound perfectly fine, they just sound different than mediums.
Effects on the Guitar
Acoustic guitars are designed with thin, flexible tops to provide quality sound. The bridge is installed in a manner that affects top vibration as little as possible. Guitar strings produce an enormous amount of tension, which increases with the use of heavier gauge strings. A number of manufacturers don't recommend the use of medium strings, as the instruments would require extra reinforcement which could adversely affect sound.
Which is Best?
The choice between medium or light acoustic guitar strings is ultimately a matter of preference. Each has drawbacks and benefits, which should be weighed carefully by the player in an effort to make the correct choice.