Theoretically, the same volume of air is used to dissipate heat. However in real applications, each arrangement has advantages and disadvantages. Air that is drawn into the fan flows laminarly. Laminar flow allows for a uniformly distributed airflow velocity in the enclosure. This is important in eliminating stagnant air and hot spots.
Air exhausted from the fan is turbulent.
Heat dissipation in a turbulent airflow can be up to double that of a laminar flow with the same volumetric flow rate. But, the turbulent airflow region near a fan exhaust is limited. Developing a well defined airflow path through the whole enclosure is essential. Vents should be at least 50 percent larger in area than the fan opening. Care must be taken to eliminate air recirculation in a fan.