Okay, so, first of all, Eelektross, with levitate, has no weaknesses and solid all around stats, not to mention a huge movepool. Levitate gives him exceptional defensive versatility, too, in contrast to galvantula who is weak to fire and rock, which are common offensive types among the late game bosses. Sluggish speed does hamper eelektross when quick hits are demanded, this is true, but when it comes to counter attacks and switch ins, eelectross does a much better job because of his no weakness and exceptional stats. Galvantula is a great pokemon, but in the context of the late game bosses at least, eelektross is more helpful and less likely to get swatted by beefy pokemon. Galvantula is more of a special attacker and speed control pokemon, and is pretty fragile if it isnt used correctly, and eelektross is more versatile, and has utility via its huge movepool. Its honestly up to your playstyle!