It is better to stagger them. You want the largest overall effective propeller area for efficiency. This is the same reason that airplanes with wide wingspan are more efficient. Note that sailplanes, where efficiency really matters, have wide and thing wings.
The reason is that lift comes from momentum, which is mass times velocity of the air that is pushed down. However, the power it takes to move this air is proportional to the square of velocity. Therefore you want to push a lot of air a little, instead of a little air a lot. Both produce the same lift, but the former takes less power.
Rotors spread out will push more air, which therefore doesn't need to be pushed as fast, which saves power.
There is one minor advantage to stacking counter-rotating propellors, which is that you can cancel the spin introduced into the air, which is just wasted motion. The Soviet Bear bomber is a good example of this. However, the spin induced in the air from a single propellor represents a relatively minor amount of power. The usual reason for stacked counter-rotating props in aircraft is for compactness and being able to increase the power imparted to the air over the limited area. That was the real reason for the configuration of the Bear. They simply needed more power to push a heavy aircraft at the speed they wanted, and more propellors sideways would have been difficult for other reasons. The Bear is a interesting lesson in what you can do with pushing propellors to the limit because you suck at jet engines.