My money is on the B787-10 because it is newer technology with lower fuel burn/emissions and commonality with the -8 and -9 models. Increased flexibility for crewing is another factor. Boeing know that if they don't proceed with the -10, then that will give Airbus an opportunity to snare market share in the 350 seat category with the A350XWB.
The B777 is 1990's technology and the only way that Boeing will be able to keep it in production is to heavily modify it with B787 era technology.
The other possibility is that this order could be the much talked about giant order for common (QF & JQ) single aisle jets.