@skedguy -- the 789 only costs less per trip assuming one uses a 3-3-3 configuration. This benefit grows smaller when one finds that the 359 has both A) longer usable payload range (meaning it can carry a higher amount of people over longer distances without facing payload restrictions), B) that it has a larger cargo capacity (important for something like a 77W/330 replacement), C) type commonality with a larger sibling (the A350-1000).
Although there were "significant" cancellations on the A350-1000, there are still over 200 orders for it (compared to 175 for the 787-10). Although they may be without the A350-800 in the family, the 359 and 3510 can cover the same market as the 777-200(ER/LR) and the 777-300(ER), with near identical seating capacities.
This also fails to account for the fact that the 787-10 (a potential 77W replacement) is less capable of flying the 777-300ER routes that MS currently flies. As Tim Clark noted when talking to the Seattle Times, the 787-10 is underpowered when it comes to operating in the hot (hot!) conditions of DXB (and a similar issue occurs with CAI). Because of the A350 (both -900 and -1000)'s higher MTOW, it means that both aircraft can take off with fewer payload restrictions compared to the 787.
This also doesn't account for the fact that there is some (minor) similarity between the A350 and the A330 that are already operated. Despite the fact that the A350 has different cockpits, fuselage, engines, and wings, the internals (fuel pumps, hydraulics, APU, wiring, etc.) is mostly borrowed from the A330. This also means that the aircraft operates "conventionally," making it easier to repair compared to the uncharted territory that the all-electric 787 offers.
So yes, the 787 may have more technological innovation. Yes, it may offer superb engine efficiency and more composites. But when it comes to choosing an aircraft, both seem to be relatively equal. The 787 is better when it comes to overall innovation and efficiency, whereas the A350 pulls ahead when it comes to physical capability and, to a lesser extent, innovation over current types. Both can use what's roughly the same engine (Trent 1000 versus Trent 1000XWB), and both are capable of taking over MS' current route structure. So, really, it comes down to which aircraft will be offered for less.