With the mask you must still use the plastic nebulizer cup that holds the saline or medication, so I don't know if that would be any different.
When I asked my pulmonologist about mask vs mouthpiece, she said she thinks the mouthpiece is more effective for adults because the drugs are delivered directly to the lungs. Using a facemask results in a portion of the medication depositing in the nasal passages, and some loss of medication due to the volume of air in the mask.
My kids' pediatrician (MANY years ago) had them transition to the mouthpiece as soon as they were able to hold it and close their lips to make a seal.