How does an airline, when buying aircrafts, decide on which engine to use? In many cases, although there are several options to choose from, the engines do not differ much in their operating parameters.
As an example I would take the Boeing 757-200. This aircraft was able to be equipped with Rolls-Royce RB211 and Pratt&Whitney PW2000. Both engines have very similiar performance ratings and also do not differ much in their physical parameters (e.g. diameter, weight).
From this perspective, it seems to me, that engine decisions are mostly driven by business consideration (e.g. sales discounts, maintenance contracts...)?