Make sure you are clear about what you are talking about. There are nasal pillows that seal against the nostrils directly, and nasal masks that are similar to full face masks, but stop at the upper lip rather than below the mouth, thereby not covering the mouth.
As a general rule, with a number of exceptions (some of which populate this forum), nasal pillows tend not to be preferred by patients at higher treatment pressures. I am not aware of any high-pressure issues with nasal masks, however. I personally use one, and my pressure gets up to 19-ish on occasion.
Both types will exclude the mouth from the pressurized airway system, thereby requiring that the patient keep their mouth closed while sleeping to prevent leaks out the mouth.