The bigger one is 50mbits/sec XAVC-S.
The other one is 25mbits/sec AVC.
I can't see any difference, except maybe a tiny change in colour.
Both are Sony FDR-1000V camera.
The reason for using the 25mbits/sec mode is that (1) you get 2x more data onto the SD card and (2) the files are 4GB chunks and can be transferred over wifi. The XAVC-S file is in one piece and seemingly won't transfer over wifi (one has to use USB). OTOH the lack of breaks makes video editing easier.
Both are 1080P 50FPS. My feeling is that the difference between 25mbits/sec and 50mbits/sec is going to be very hard to see, especially is very rapid movement is avoided.