I voted EV training. Before I get started on EV training, I infect the Pokemon I'm training with Pokerus, then feed it the necessary vitamins (provided I can afford them). Then I give that Pokemon the appropriate Power item, then head to an appropriate area where I can EV train that Pokemon (i.e. I go to the Old Chateau if I want Special Attack EVs, since Gastly are the only Pokemon in the area (or Haunter/Gengar with a GBA game in the GB slot).