Firstly we will explain the similarities. The headgear, strap and elbow connection point are the same with the two masks. The difference between the two masks is in the cushion.
The P30 (pillow) will make a seal inside your nostrils, whereas the N30 will make a seal underneath your nostrils.
The beauty of this mask is that the P30 and N30 cushions are interchangeable. So if you’re already on a P30i or an N30i, you can use the other cushion with the other frame and headgear.
So in summary, the difference between the ResMed AirFit P30i and the N30i is in the cushion. P30 inside the nostrils, N30 a little cradle that sits underneath the nostrils.