We use X-ray scattering to investigate morphology and dielectric spectroscopy to study ionic conduction and dielectric response of imidazolium-based single-ion conductors with two different counterions [hexafluorophosphate (PF6-) or bis(trifluoromethanesulfonyl)imide (F3CSO2NSO2CF3- = Tf2N-)] with different imidazolium pendant structures, particularly tail length (n-butyl vs n-dodecyl). A physical model of electrode polarization is used to separate ionic conductivity of the ionomers into number density of conducting ions and their mobility. Tf2N- counterions display higher ionic conductivity and mobility than PF6- counterions, as anticipated by ab initio calculations. Ion mobility is coupled to polymer segmental;motion, as these are observed to share the same Vogel temperature. Ionomers with the In-butyl tail impart much larger static dielectric constant than those with the n-dodecyl tail. From the analysis of the static dielectric constant using Onsager theory, there is more ionic aggregation in ionomers with the n-dodecyl tails than in those with the n-butyl tails, consistent with X-ray scattering, which shows a much stronger ionic aggregate peak for the ionomers with dodecyl tails on their imidazolium side chains.