The corrrect answer would be: En route fuel stop only if necessary, and that's the case for fully loaded freighters.
Pax payload is less than freighter payload, thus a 14hr non stop flight is conceivably more cost efficient.
It wouldn't matter if the fuel burn were less with an en route stop, because the flight has to be cost based; not just "fuel-cost" based.
It would be difficult to even the cost savings when an airline would have to spend money for hotels, landing fees, crew salaries, ground handling, line maintenance, airplane wear and tear [cycles, brakes] . . . just to reduce fuel burn.
And who would fly on your airline from HKG to LHR via DXB if other airlines would fly nonstop?