I think Dark PLOX techs in Weavile...?
Anyway, you use it to deal extra damage with Special Darkness Energies as well as to take out Gengar faster (Gengar's weakness is Darkness). Which is better probably depends on your metagame. If you see a lot of Gengar around, then use Dark PLOX. Otherwise, you can use the space that the Darkness Energies and Weavile took up to tech in something else.