If I had to choose between these two, I would go with Manectric, as long as we're talking about xy or oras, otherwise, go with luxray. However, an electric type Pokemon that I've found to be very useful is Raichu, but only after it learns some good moves while it's Pikachu. Or maybe you want a brutish electric type Pokemon, then you should try Electivire, just make sure you like his stats, ability, and EVs before you trade him into an Electivire. My final answer would be Raichu or Manetric(with megastone).