720 is always Progressive, no matter what frame rate you use.
PN vs P is just how the progressive frames are written to the cards. WIth PN (N=Native) you only record the actual frame rate, so if you set the camera to 30PN it only records 30 Frames, 24PN only records 24 Frames. If you set the camera to 30P the camera records each frame twice, so you have 60 frames but only 30 different frames. 24P records every other frame either twice or three times. So there are 24 unique frames but some are recorded 2 or 3 times (2:3 pulldown) these redundant frames can be removed on transfer into the NLE program.
So why record in PN, simple you got a lot more record time on a P2 card. Any “p” framerate gets you about 1 min/GB where as 30PN gets you 2 min/GB and 24PN gets you about 2.5 min/GB.
But 720P@60P gives you 60 unique frames and would probably give you the smoothest motion.