I am an economist, and the way we would evaluate which engine is superior is which airplane sells for a higher price. The all-in price of the plane should incorporate all of the engine considerations mentioned (initial cost, fuel cost, maintenance cost, thrust, etc.). One would only pay for plane equipped with XX engine versus YY engine if on net, XX engine is more economical.
While I am certainly not an expert on this, but it is my understanding that RR-equipped 757s sell at a higher price than PW-ones. While this is not definitive as to whether 2-spools are better/worse than 3-spools, it does suggest there are aspects of initial cost, fuel cost, maintenance cost, thrust, etc. in which the Roller must be better than the Pratt (unless the airline CEO just likes the look of interlocked Rs on cowling better than an eagle)
Of course it is also possible that different airlines' demands (lots of short hops and high cycles vs. long haul) could make Airline A find engine XX to be most economical while Airline B finds engine YY to be most economical.
Since both 2-spool and 3-spool models have been successful, it seems the latter explanation is likely.