Eyes Only
You post read as though you were saying the A350/380 were a common endorsement and not the B744/8. I don't advocate the B777 as common with the B744/8 just as the A330 is not with the A350/380.
Bottom line, I don't think the commonality, or lack of, will be of overiding significance if one type over the other has better economics of operation. I personally see CX eventually purchasing both types as the route structure (Slot constrained airports etc) and type of CX operation phylosophy(high frequency with cargo belly space) being the driving factor. Even with a minimal order of A380s and -8Is they would both be part of a sizeable fleet due to the A350s and 744Fs/-8Fs.
As for noise levels, until recently I would agree with you hands down...but...ever been in an ERF, could hardly believe it was a Boeing and I hear the -8F/I is quieter. I look foreward to the chance for a trip on one (not easy as a frieghter) to see (hear) for myself.
I believe Air China also ordered 20 -8Is last August. It seems to have been kept very quiet but that should start the rumour mill going what with the Cx tie-up.