The only thing that blows up more creeper than it is a thor. That thing is night and day difference in your ability to clear creeper. How much more creeper than a mortar does this thing clear? 4 times as much?
The mortar firing mechanics can be found here:
http://knucklecracker.com/wiki/doku.php?id=cw1:units:mortar (http://knucklecracker.com/wiki/doku.php?id=cw1:units:mortar)
The mortar shot can clear up to 144 "units" of creeper, a drone shot can clear 200.
Drones are more effective as they can usually be targeted at very deep creper, maximizing damage.
Optimal damage/energy for drone is 125 creeper per shot and about 42 for mortar.