It's up to you, really - both options have their advantages and disadvantages. Pikachu tends to be kinda fast already as well but if you're talking competitive you'd want to EV train regardless.
Or you could get another Pikachu via breeding so it'd still have volt tackle and evolve one of them so you have both options. =P Anyways, use the Help thread next time. I don't see too much point with this since there is no real set answer and I feel you'll just get mixed responses, so thread closed.