Any airline choosing a new aircraft will have performance guarantees in the contract. This includes range, weight, speeds, fuel burn etc etc....
A350 service date is 2010 I think, the 787 goes into service before, although the 787 doesn't come to AC before 2010 - so at least it will be in service and glitches ironed out - as there always are when a new type enters revenue service, no matter the exhaustive testing.
Don't know how many A350s have been ordered...but I think it's miniscule compared to the momentum 787 has gotten right now. If the A350 doesn't yield more orders, one might assume the programme might even be canceled - not the first time it's happened.
Comments anyone ?