Party pumps dispense beer by use of outside air pumped into the keg via a hand pump.
This type of dispensing is appropriate only when the beer will be consumed within a few hours, because the outside air will cause a bacteria reaction that will spoil the beer within 8-12 hours.
Keg couplers dispense by use of pressurized gas from a gas cylinder. For ale and lager type beers, Carbon Dioxide (CO2) is used. For stout type beers, a mixed 75% Nitrogen / 25% Carbon Dioxide gas is used.
This type of dispensing will allow the beer to remain fresh, if kept at the proper temperature and pressure, for 45-120 days.