I fly both, B777 and B787. To be exact, B777-300ER, B777-LR, B777-F, and Boeing 787-9 and now the B787-10, and I have flown the B787-8 during the training period 5 years ago.
The fuel on a standard trip from middle east to say Washington IAD was compared. B777-200LR was 125 tons whilst a standard load on the was 81 tons, burning 76 of those tons to reach IAD and the rest, 5 tons is alternate fuel and reserves. The B787-9 flies the route 40 min faster for the ULR flight time of a 14 hour flight than the B777-200LR. Mach 0.86 against M0.835/.84. Our B777-200LR carried 240 pax in 3 class whilst the B787-9 has 235 in 3 class, or 299 in 2 class and the B787-10 has 336 in 2 class.
This is the range limit of the B787-9 to take off at max 250836kg and fly 14 hours with a full load of pax, whilst the B777-200LR can uplift more fuel and still carry on another 4-5 hours of flight time on a trip like this.
The point is, the B777-200LR was not made to to fly short trip like this, but rather those 18 hour trips over the pole, with a load, whilst the B787-9 would fly far on full tanks of 109 tons fuel, but without much load. Its really difficult to compare these two types as they have different jobs by design.