Sailplanes vs flying wings
Most often, a UAV builder chooses between having a tail with long, skinny wings that will fly slowly very efficiently, and having a fat-winged lifting body with no tail that can fly slowly or quickly, a bit less efficiently. Being able to fly slowly is a prerequisite to landing safely in most circumstances, and the long chord of the flying wing makes its near-stall sink rate very small.
Both sailplanes and flying wings have achieved significant endurance flights, with sailplanes staying in the air longer, but flying wings typically covering more ground due to their greater speed.
Two other factors come into play, however:
The first is steady wind penetration. A sailplane that can not make easy progress into a headwind is a sailplane that may not make it back to home base. Flying wings, with their typically wide speed envelope, can both make it home and land when they get there. The efficient, skinny wings on a foam sailplane are floppy, creating problems at high speed unless they are significantly reinforced, while a foam flying wing doesn't have this problem to the same degree.
The second is turbulence. Most UAVs are flown inside the atmospheric boundary layer, where tropospheric steady winds are broken up and made gusty by ground obstacles. Sailplanes are made to self-stabilize even when flying extremely slow, so if the wind shifts in direction, they will automatically pivot into it. Their tail provides a higher degree of leverage on the horizontal and vertical stabilizers than a flying wing does, and as a result the flying wing is steadier in turbulent crosswinds. This is only enhanced by the increased speed possible with a flying wing.