A randomized controlled trial published in the journal Sleep & Breathing reported that bilevel positive airway pressure (BiPAP) reduced the apnea-hypopnea index to 2.3 per hour while continuous positive airway pressure (CPAP) reduced the apnea-hypopnea index to only 4.4 per hour. However, the difference was not statistically significant.
How Stuff Works explains that CPAP sends a constant stream of air down the user's throat. This is effective in keeping the airway open and thus prevents sleep apnea, but it can be uncomfortable for the user because the user has to exhale against the pressure of the incoming air. BiPAP machines reduce the air pressure during exhalation which makes the device more comfortable for the user and has a lower rate of side effects such as sore throat.