This is from a forum thread:
we’re looking for the best possible video out of the RX100, we should use 50i 24M for 25fps video and 50p 28M for 50fps video.
In theory, shooting 50i 24M gives us the best bitrate per frame in this camera, almost twice as much as shooting 50p 28M.
Shooting 50fps however would have neighboring frames changing less than when shooting 25fps, helping the encoder do a better job, but still the per frame bitrate is lower, and here’s a comparison that shows is. If you look at the darker areas in the back where the window is, you’ll see that the 50i version is slightly cleaner.