The headrest can be used up or down. When the headrest is up, the carrier is approximately 3" taller than the regular body size. Many people prefer to have this option for their baby. I suggest choosing the headrest if you plan on using your BabyHawk with a newborn or if you like having your baby sleep on your back often -- it will give him proper head support.
Most older babies would rather be able to see out easily and have their arms out of the top of the carrier, which is easily done in the regular body size.