The Catalyst works somewhat like a Focus on other games. It come in handy when the are trying to keep their powers under stricter control, perhaps granting an Edge on an Action Roll or being depleted for +1 when Resisting Consequences coming from a failed action while using these powers.
Juice is a more open idea of concoctions that you can drink and gain momentary powers, like a magic potion. How it presents itself is to be decided in fiction, depending on your game's tone and premises. For example, it can have a variety of previously set powers (and flavors) to choose from, or it's tailor-made according to the needs of the customer, or even grant a random power based off the user's genes.