You never want matching airflow in a PC case, it creates pockets of heat, frying components, usually the top disc drive or sometimes the PSU. Positive pressure

You never want matching airflow in a PC case, it creates pockets of heat, frying components, usually the top disc drive or sometimes the PSU. Positive pressure is very good at assisting the PSU fan and any radiator exit fans you have, negative pressure is very good at cooling the edges of the MoBo and upper drives in a tower system. Negative is generally cheaper, so its usually negative.