I find Will-O-Wisp easily the better option because of two things:
Passive Damage
Only needs to be used once
In a metagame as fast as DP OU, I simply do not want to have to waste turns by using charm over and over again. Will-O-Wisp allows me to do it once and then I know it will be crippled for the rest of the match.
The passive damage is quite excellent in OU. With all of SR and SS around OU, even will-o-wisping a wall will allow a much easier time for your sweepers to "sweep." Even though its only 12% damage per turn, SR only does that every switch in.
Simply put, charm has it's uses, but the benefits of a lasting effect on the opponent's Pokemon are just too good not to be used.
Passive Damage
Only needs to be used once
In a metagame as fast as DP OU, I simply do not want to have to waste turns by using charm over and over again. Will-O-Wisp allows me to do it once and then I know it will be crippled for the rest of the match.
The passive damage is quite excellent in OU. With all of SR and SS around OU, even will-o-wisping a wall will allow a much easier time for your sweepers to "sweep." Even though its only 12% damage per turn, SR only does that every switch in.
Simply put, charm has it's uses, but the benefits of a lasting effect on the opponent's Pokemon are just too good not to be used.

