In both the cases, the molecules of gases move across the hole. Then what is the difference between them. Also, will effusion occur if pressure on both sides of the hole is equal.
The transport of the gas through the hole is described well by diffusion if the mean free path between collisions is small compared to the characteristic length of the concentration gradients. If this is not the case you have effusion.In general, the efflux of a gas or fluid can be more complicated. See, e.g., Torricelli's Law