...even with the Butterfree version of Miltank, I would argue that going first is better. You don't know that you'll be able to get a Butterfree out on turn 1, and therefore it's far better to take the safer route, give yourself an extra turn to set up, and decide to go first. Same thing with other aggressive decks like Plasma or Big Basics--going first is not only safer, but the benefits you gain from the extra turn far outclass the possibility of getting a turn one attack.