Fleet decisions are made with various factors in mind - the most important being mission, the routes and services that an airline plans to operate with an aircraft.
Remember, every airliner costs tens if not hundreds of millions of dollars each which must be paid, financed or paid via lease payments, so a huge consideration is cost - if an airline is looking for 50 airliners in the 150 seat category, the price that Airbus will accept for its A320s and the price that Boeing wants for its 738s will be a huge factor. For example, if Boeing is willing to make a better deal than Airbus, the airline may go with Boeing even if that airline had operated only Airbus to date (its also true the other way around). Also, delivery schedules are very important, if an airline is looking to acquire new aircraft, they must settle on a delivery schedule that is suitable.....if an airline want to open a new route in 6 months time, Boeing could have the best plane around, but if it cannot be delivered for 2 years the airline will look to Airbus, lessors or the 2nd hand market. There are other factors, such as leasing deals, residual values, operating costs, training, spares, etc. But, in the end, an airline will go for the best aircraft at the best price.
selected a mix of 777 and A332 aircraft recently, NW
went with the 753 and A332/333 to replace the DC10s, BA
flies Boeing long-haul but is going Airbus for short haul, while SAS flies 737NG short-haul but Airbus for long-haul. It keeps things interesting.