I believe "On fall" will only trigger once the Sprite begins falling, whereas "Is falling" constant triggers whilst the Sprite falls.
Say you have a variable that starts at 0, and you use "On fall- Add 1 to variable", and make the Sprite fall, the variable would only reach "1". Whereas having "Is falling- Add 1 to variable", the variable will continue to go up until the Sprite is no longer falling.
Hope this helps!