msbbarratt stated:
What's we'll never know is whether the Trent 8115 would have made it better still....
AFAIK GE's big twin spools weigh a lot more than RR's big trip-spool equivalents; that sounds like a bad thing for everyone except GE.
RR's tend to have better climb fuel burn due to the triple spools, whereas GE's tend to have better cruise fuel burn. So for shorter stage lengths, the Trent 8115 likely would have held the advantage, whereas the GE90 would do better on longer stage lengths.
It's quite interesting to note that the 787 launched with RR. The airlines presumably didn't want to be denied the choice this time round.
The 787 was always going to have two engine choices and RFPs were sent to GE, RR and P&W.
The original A350 also would have had two (Airbus agreed to give GE the first two years of production to develop an engine). Once Airbus decided to go bigger, that effectively killed GE's interest (and I assume Airbus was as wary of a large-thrust P&W GTF on the A350 as Boeing was having it on the 787).