good question...this ques i believe all students preparing for jee asks.....see first ans comes directly from gauss law...right???now the second part / is the electric field of a CONDUCTOR at any point infinetly close to the surface...now what is a conductror???a body with charge uniformly distributed through out..that ans y thereis charge on both the sides....secondlt it is not valid only for plate but for any conductor bt at a point really close....

 

TIP:second formula is rarely used..1 in 1000 times so if u r confused its a safe bet to go with the first one.....