This depends on several factors:
Two small smartbombs will require less powergrid than 1 medium but obviously one more slot.
You can fit two different damage types with 2 small smartbombs which could be a good or bad thing but more likely to be a good thing.
The biggest issue for me with smartbombs is their range and small smartbombs suffer badly here although most drones should orbit you within their range.
Missiles can work too but unless you are using FoF (Friend or Foe) missiles then you have worry about targeting several drones and given their size take a while to complete lock.
A smartbomb is generally more effective against drones as they'll all get damaged at the same time and, unless your attacker isn't paying attention, he'll be forced to withdraw them which is almost as good as killing them.
Also bear in mind that your smartbombs will also damage your own drones if they return to orbit you.
Also relevant is the risk of your smartbombs damaging a neutral ship or object (station, gate, even a can that isn't yours) in high-sec space which will bring CONCORD into the fray.
Turrets don't work so well on drones due to their very high transversal velocity (the speed which the object travels across your field of view) and very close range so a webber is essential there.