f1 = 
      u  /.  NDSolve[{D[u[x, t], t, t] + D[u[x, t], x, x, x, x] == 
                0, Derivative[1, 0][u][0, t] == 
                0, 
Derivative[1, 0][u][π, 
                  t] == 0, u[0, t] == 0, 
              u[π, t] == 
                0, Derivative[0, 1][u][x, 0] == 
                0, 
u[x, 0] == 1 - Cos[2  x]}, 
            u, {x, 0, Pi}, {t, 0, 6}, 
            MaxStepSize -> 0.1][[1]]

f2 = 
      u  /.  NDSolve[{D[u[x, t], t, t] - D[u[x, t], x, x] == 0, 
              u[0, t] == 0, 
              u[π, t] == 
                0, Derivative[0, 1][u][x, 0] == 
                0, 
u[x, 0] == 1 - Cos[2  x]}, 
            u, {x, 0, Pi}, {t, 0, 6}, 
            MaxStepSize -> 0.1][[1]]

Table[
      Plot[{f1[X, T], f2[X, T]}, {X, 0, π}, 
        PlotRange -> {{0, π}, {-2, 2}}, 
        PlotStyle -> {Red, Blue}], {T, 0, 3, 0.01}]