There is a relatively big error regarding the airbus A318/319/320/321 and the Boeing 737 fleets. While the boeings do have better range the Airbus have better cargo loads.
For instance United and Continental airlines are in the process of trying to merge however for now they are taking Boeing crews and sending them to Chicago and Airbus crews and sending them to Houston. Reason being is that the airbus can carry roughly 30% more cargo than the boeings. The Airbus’ hull is designed differently and can fit the cargo containers in them. This is the main reason United wants them in Houston so that they can service Latin/South America.