The main thing to consider when considering Source v Destination is that the Source store can trigger inventory updates to the Destination Store on both manual adjustments and orders. A Destination store only triggers adjustments when an order is placed and fully paid.
Syncio is designed this way as the Source store is the store that holds the master inventory and is the store that fulfills and ships the order to the customer.
The Destination store is the store that syncs product from the Source store to list and sell. Orders are then passed back to the Source store to drop ship to the customer.