Defensive clefairy (to; dondon)

I am not dondon, but I have an opinion on nearly anything, so here goes...

First I will assume DG means DV.

There is very little difference between the charge moves, so there won't be a noticable difference.
That said, Clefairy does not have the stats to make a good defender.

https://pokemongo.gamepress.gg/pokemon-move/zen-headbutt
https://pokemongo.gamepress.gg/pokemon-move/pound
You don't need mathematical hero, just take a look at those pages, the defensive dps of ZH is higher than that of pound, the number was obtained from a gym-defending simulation by gampress I believe. Also the general rule for quick moves when defending gyms is, the quick move with higher damage per hit but longer cooldown deals more damage than the quick move with lower damage per hit but shorter cooldown. This is because when defending gym, PoGO AI will play the defending mon, not human players, and the quick move isn't used as soon as the cooldown expired, in other words, the quick move isn't used as many as it could have been with human players, so the maximum possible dps of the quick move won't be achieved by the AI, therefore quick moves with higher damage per hit would deal more damage when defending gyms.