Kiting = Most efficient.
Only one player is required to deal with the adds and they should not be taking any damage from the adds, only damage from Magmaw's abilties which hit the entire raid. This has the lowest DPS allocation to dealing with the adds as well as the lowest healing allocation.
Killing = Second best.
Using primarily ranged DPS to kill them quickly, generally with the goal of killing them before the next wave spawns. You never leave adds up so there is never a concern with loose adds. Killing units is the most effective way of dealing with adds, however this does mean the fight will last longer since DPS is being pulled off the boss. Generally pets are used to help hold the adds in position like Hunter Tenacity pets and Warlock Felguard because pets can not be infected by the adds.
Tanking = Weakest method.
This is the worst method because while like the kiting method it only requires one player, it has one major difference, that player is taking damage from the adds. Generally speaking doing the tanking method, this off tank will take close to two thirds the total damage of the main tank, effectively increasing the healing requirement of the encounter by around 30-50% (you are taking damage that is unnecessary). While this method may be easy for groups farming the content, groups just starting it will have a hard time keeping up with the higher healing requirement. This is why you CC trash in heroics, but then once you start to outgear it you stop CCing, taking extra damage becomes less threatening as you reach the point where you can deal with it.
For a group that is just starting raiding content you should either be kiting or killing the adds. Tanking them will put too much strain on your healers, especially when first learning the encounter.
"In anything, if you want to go from just a beginner to a pro, you need a montage." /w TankSpot WTB Montage for Raiders.