Pokemon EV training help?

Ok, well I am making a some-what competitive team and one pokemon on my team is porygon z. I want to ev train this pokemn except i dont know if it would be better to train it as a regular porygon, or should i just evolve it to a porygon z and then ev train it. Because if i evolve it by trading using an upgrade and a dubious disk, it will still be level 1.