Being somewhat vexed by this question I have done a bit of research. The reliability of the Rolls engine is very good, An Icelandair 757 with an RB211-535-E4 clocked up world record 40,531 hours on the wing recently.
However the RB211 is heavier and does burn slightly more fuel, (comparing like with like, 535-E4 with PW 2037), and the old beancounters are not going to take that unless there are other compelling reasons
The crunch may be in the take off performance. At 250,000lbs, sea level, standard day, the RR engined 757 requires 6800ft of runway. The PW2037 requires 8700ft.
(Assuming I have read the graphs corrrectly, by no means guaranteed, it's been a year or two since Perf A)
I suspect that on European routes, that field requirement is probably unacceptable. The climb performance is presumably similarly disadvantaged and would I suspect make the PW aircraft next to useless for a charter operator.
Info is
Here
Anyone got any better ideas?