usually, first is better, but with some decks (such as denial decks, ESPECIALLY with pest) it is better to go second. I believe the reason somebody said going second is better for a deckout deck is because deckout decks are generally made to not get decked out themselves. In simpler terms: a deckout deck does not depend on going first to win. This means that if they go second, they have a (slight) card advantage, possibly allowing them to get their stall cards out easier.
As stated above, going second also gives you the element of surprise, especially if towers are being used.
then again, speed decks that rely on supernova get a huge advantage from going first, as do many other kinds of decks.