Conducting polymers represent an ideal class of materials to bridge the worlds of biology and microelectronics. Their soft, often gel-like properties can support efficient ionic and electronic transport, appropriate for transducing biological ionic fluxes into electronic signals, or vice versa. Recent bioelectronic devices have relied on conducting polymers such as poly(3,4-ethylenedioxythiophene) doped with poly(styrenesulfonate) (PEDOT:PSS) due to its ease of processing and commercial availability. Regardless of the device, format, or application, such materials should be efficient mixed conductors. A figure of merit, however, depends on both the device and application. Organic electrochemical transistors (OECTs) for example, have been shown to be ideal platforms for neural interfacing in vivo and in vitro due to their high transconductance (>1 mS). By tuning device geometry and materials processing we show that the materials figure of merit for the conducting polymer channel is the product of the electronic charge carrier mobility and the volumetric capacitance (C*), in stark contrast to field effect transistors. C* is directly related to the ability for ions to easily penetrate and drift within the film, which, along with high electronic mobility confirms the need for efficient mixed conduction. Most conducting polymers, however, are optimized for electronic transport and optical absorption. In this work, we study the effect of materials chemistry and processing on both ionic and electronic transport. Using resonant scattering, electrochromic moving front and charge transport studies, we show that molecular level interactions and meso-scale ordering/purity affect both types of transport, and that mixed conduction can thus be tuned using processing and formulation. These findings help to formulate general device and materials design rules, which allows us to put a broad range of potential active materials (varying chemistry, formulation and processing) into a common context in order to advance bioelectronic materials design.