When choosing the correct fan option there are two confusing, yet very important parameters that you need to decide on. Air Flow and Static Pressure
Airflow: This is the maximum amount of air moved by a fan when there is no resistance, such as open air.
Static Pressure: This is the maximum pressure produced by a fan when this is maximum resistance and no air flow, such as in a sealed box.
Generally, for a wide open application, airflow is more important.
For a high resistance application, such as a compact space or a circuit board with many airflow blocking components, the static pressure is equally, if not more, important.
However, the most important thing to note is that these values by themselves are essentially meaningless. Both are maximum values set in ideal environments, which most practical applications will not meet. Which is why manufacturers supply a fan curve chart like the one below.
These charts show the relationships between Static Pressure and Air Flow. Using these curves will reflect a fans variability in real world applications.