Hybrid surfaces are interactive systems combining techniques of direct-manipulation multi-touch surface interaction with elements of tangible user interfaces (TUIs). The design space for such complex hands-on computing experiences is sufficiently broad that it can be difficult to make allocation of function decisions and decide when interface elements should be given either a physical or digital instantiation and the extent to which different interface functions should be made to model real-world interactions. In this paper we present two case studies of hybrid surface systems we are developing and discuss how we have reasoned through these kinds of design decisions. From this we derive a set of observations about properties of physical and digital elements, and offer them as a design resource.