If you shoot 24pN, only the native 24p files are recorded and the files have a frame rate of 23.976.
If you shoot 24P, the files are recorded along with duplicate frames in a 60p stream, and the files have a frame rate of 59.94. You need to remove the duplicate frames to edit as native 24p.
There is no difference in visual quality because each frame is exactly the same between the two. The only difference is that in order to fill out the 60p stream, the codec makes copies of frames and inserts them into the streams.