i think just to clarify, the cockpit/flight crew don't choose a route. the flight planners in waterside plan the route - the flight crew can question it if they see something they don't like, but most of the time they are just given a route and that is the one uploaded and used.

the a380 and 777 have different performance in terms of altitudes and cruise speeds which may mean that the optimal routings for each are not the same. those two flights may also have different loads - a lightly loaded flight can get up higher more quickly usually which may mean a different routing is better as they can take advantage of a tailwind at a higher flight level. and of course changes in forecast winds over that 2 hour time difference could mean different routes are favoured even over that short timespan. and you need to have diversion points along the route, which for an a380 is not easy as you need airports with cat10 fire coverage - 777 needs cat 9 i believe. so lots of factors - and many i probably haven't even mentioned (e.g. airspace capacity constraints etc.).