Diff is to help keep as much traction during tight turns - as in track racing.
Slipper is to maintain equal power front/rear which is most needed when bashing.
Here is a decent link I searched and found here on the forums.
Hope that helps.
Slash 4x4 - HCG
Kraton - Basher (4s)
Raider Mega ('14) - strong chassis/weak electronics
Dromida Ominus - consistent and dependable.