It's a good question raised by the OP and the answer gas been nodded at. The reality is that airlines take into account availability, money, support, spares, training, leasing costs etc. before deciding. These vary from airline to airline and from country to country and they depend on where you are starting from. Manufacturers may also provide individual performance guarantees, like round-trip fuel, no de-icing due to cold soak, runway performance etc. Not surprisingly, pilots rarely make these decisions (too many idiots amongst their ranks, as can be witnessed from reading some replies to various threads). It's the job of the sales guys to help the buyers do the sums - in the process helping their own product to appear to be better value for money. In times past apparently some of these discussions took place in establishments of "ill repute" and others write the reasons why their products should be chosen on notes inside brown envelopes full of cash.
These decisions can take a long period of time because rarely do you change aircraft type more often than once every eight to ten years. Getting it wrong can be very expensive.
PM