Our department has a storefront, do I need to switch to ARO billing?

No, you could have both options available dependent upon your as needs.  While both are a means to receive revenue for your department storefronts allow you accept payment up front, usually for one-time events.  ARO billing should be used for items that require billing on a regular basis (monthly, quarterly, annually) or is part of a contact or MOU to be billed.