Kick reduces the recoil when you shoot your weapon so more of the bullets land on target. On a sniper rifle when you scope in, the gun will sway back and forth. Using stability cuts down the back and forth sway. Using stability on a sniper was just an example of course you can use it on the other weapons as well.
The only deciding factor is which proficiency is best for a particular gun and play style. Makes for more complex setups.