When to use s=ut+1/2at² and when 2as=v²-u²

The equation $s=ut+\frac{1}{2}a{t}^2$ is known as the second equation of motion and it is used to calculate the distance travelled (s) by a body in time (t), ​the body having initial velocity (u) and acceleration (a). This equation has four values in it, so if any three values are known, the fourth value can be calculated. 

The equation $v^2=u^2+2as$ is known as the third equation of motion and it is used to find the ​velocity (v) acquired by a body in travelling a distance (s), ​the body having initial velocity (u) and uniform acceleration (a). This equation has also four values in it, so if any three values are known, the fourth value can be calculated.